Wrapping things up, Mr Aiken tells us it's been a "very long day dealing with a lot of material"... and he can certainly say that again!

Inquiry chair Sir Patrick Coghlin says with a smile that it's been "completely fascinating".

We'll be back tomorrow from 09:45 for more insight into how things developed during a key period for the RHI scheme.

But for now we're off home for a lie down that's been well earned, even if we do say so ourselves...

BBC News Northern Ireland

A letter sent by Arlene Foster to banks was the subject of "media speculation" and "innuendo", the inquiry was told.

Image source, PA

It heard that in fact it had been written by one of her officials and was sent to banks without amendment by the then DETI minister Mrs Foster.

The letter was intended to encourage banks to lend to businesses who wanted to invest in renewable technologies but were facing resistance from the lenders.

DECC cut the subsidy on offer for medium commercial biomass boilers by 5% and it outlined its reason for doing so in the press notice.

It said uptake of that size of installation was "exceeding the rate we expected when the tariff was originally set" and that suggested that "the tariff is higher than is needed to incentivise installation".

Image source, RHI Inquiry

"We may be overcompensating further installations if we do not adjust our tariff downwards."

That surely should have raised questions for DETI about its own scheme, Mr Aiken notes.

DECC issued a press notice in May 2013 that was sent to DETI along with the letter from minister Mr Barker.

It was lodged in the DETI electronic filing system the same day and it advised that the digression mechanism had been triggered.

Image source, RHI Inquiry

Mr Aiken notes that it also contained "a rather stark description" of the purpose of tiering.

It said: "The biomass tiering system is designed to prevent the production of heat solely for the purpose of claiming the RHI tariff."

As of the end of May 2013, there was nothing in writing from DETI's energy team to Mrs Foster about:

• why there was no need for interim cost controls in the NI RHI scheme;
• why digression had been introduced in the GB RHI scheme;
• why a cost control mechanism in the GB scheme had been triggered.

Image source, RHI Inquiry

Mr Aiken says the inquiry will have to probe the civil servants on why "significant developments" don't appear to have been "met with a recognition of their potential significance" for the NI initiative.

At the end of May 2013, DECC minister Greg Barker wrote another letter to Mrs Foster in which he explained that his department was implementing cost control changes to its RHI scheme.

When Mrs Foster's office referred the letter to the DETI team dealing with the RHI scheme, official Joanne McCutcheon said that it was similar to the previous letter that had been received from Mr Barker and no significant action was needed in response to it.

Image source, Getty Images

But Mr Aiken points out that the "difficulty" with that was that Mr Barker was "now talking about something entirely different from that about which he wrote" previously.

The earlier letter was about the delay to opening the non-domestic RHI scheme but the latest one covered changes to tariffs and triggering the digression cost control.

Mr Aiken says that the opportunity to tell Mrs Foster about the cost control mater "was not taken".

"Did the minister not need to know about very dramatic events in the context of RHI that's going on in GB?" he asks.

In May 2013, DETI received a copy of a DECC update on the GB RHI scheme, which showed that digression of tariffs for medium-sized biomass boilers had been triggered.

Image source, RHI Inquiry

Mr Aiken notes that that was 18 months into the operation of the GB RHI scheme.

"What if anything did that mean for the Northern Ireland scheme when it reached the 18 months stage?" he asks.

Mrs Foster's adviser Andrew Crawford (below) queried what would happen if two RHI boilers were installed by a single applicant at different times, rather than both at once.

He raised the issue when he looked over the submission to the minister from DETI's energy team official Mr Hutchinson, concerning the response to the REM letter.

Mr Crawford was told by the energy officials that it depended on the length of time between the installations.

They also said that the regulations "prevents scenarios where applicants seek to install a number of smaller boilers in attempt to receive higher incentive payments".

The inquiry will have to consider what consideration, if any, DETI gave to using poultry manure to produce renewable heat through the RHI scheme, says Mr Aiken.

Image source, Getty Images

He poses the question of whether any effort was made to consider a specific tariff for poultry farmers using manure instead of wood pellets to produce their heat.

And he wonders whether any thought was given to how that could've been "better for the government, or the taxpayer".

DETI energy team official Peter Hutchinson drafted a response for Mrs Foster to REM's letter, and in it her department rejected the claim of a "perverse incentive" in the RHI scheme.

The response, sent in Mrs Foster's name, said that "I can assure that this is not the case".

"Under the regulations, where two or more plants are using the same energy source and form part of the same heating system, they are treated as a component plant and the total capacity is assessed," it went on.

Image source, RHI Inquiry

It goes on to assert that "two biomass 99kWh systems that form the same heating system would receive the tariff appropriate for a 198kWh system."

The letter advised REM to contact Ofgem - the scheme's administrator - if it had any further questions.

Perhaps a fan of '80s and '90s alt-rock, inquiry chair Sir Patrick Coghlin smiles at the Shiny Happy People in the Senate chamber and points out that REM in this circumstance is "not the band" but an energy firm.

A renewable energy firm drew Stormont departments' attention to the RHI scheme's "perverse incentive" for poultry farmers just weeks after the initiative opened.

In April 2013, DETI received a letter that had been sent by a renewable energy firm to the Department of the Environment (DoE), in which concerns about the RHI scheme were raised.

The letter had been written by Renewable Energy Manufacturing Ltd (REM) in January that year, 10 weeks after the initiative was opened, and raised concerns that the scheme was acting as a "deterrent" for poultry farmers from using its manure-to-energy system.

Image source, AFP

It said the GB RHI scheme was structured to encourage the use of poultry manure to produce renewable heat but the NI initiative had the "directly opposite effect".

It also drew attention to the NI scheme's "perverse incentive for farmers" to install several small boilers rather than one large unit in order to obtain a higher tariff that was on offer for smaller systems.

REM said there was an "evidence absence of joined-up thinking" and there needed to be a "rebalancing" of the scheme by "the responsible minister Arlene Foster".

Returning to Mr Barker's letter to Mrs Foster, the DETI minister's office had sent it to department's energy team for a draft reply to be drawn up.

By the time a submission was sent to Mrs Foster in response, the energy team would've had the benefit of the details in the Arthur Cox report on the GB scheme, including the details about adding digression to the non-domestic initiative.

Image source, Getty Images

In the submission, the energy team outlined that the department's hope was that the domestic scheme would be operational by the end of 2013.

In the reply letter to Mr Barker, Mrs Foster indicated DETI's "appreciation of the complexities of designing" the RHI scheme and she said she was looking forward to hearing more about DECC's proposals for its scheme.

Mr Aiken points out that the letter was all about the domestic scheme and nothing was mentioned about measures to add budget control for the non-domestic initiative, which was also a focus for DECC at the time.

The legal firm Arthur Cox was engaged by DETI to report on the legislation relating to the GB RHI scheme.

It informed DETI that DECC intended to "introduce digression into their forthcoming domestic RHI in addition to replacing the non-domestic interim cost control with digression in the non-domestic scheme".

Image source, DETI

Mr Aiken points out that even if DETI officials had not read DECC's original policy documents where they could find the information it was then being "summarised in a report that taxpayers' money has been used to acquire".

The message was that digression was being used in both the GB domestic and non-domestic RHI schemes.

As of mid-March 2013 - four months after the RHI scheme had opened - nine applications had been received, with only two of them, for biomass boilers, being accredited.

Later that month the DECC minister Greg Barker (below) wrote to Mrs Foster to tell her that the domestic Great Britain RHI scheme was being postponed by a until the next year.

He told her that work was continuing on the initiative and the plan was to open it in spring 2014.

In developing the domestic RHI scheme in Northern Ireland, DETI had planned to broadly follow DECC's timetable for the GB initiative.

Image source, Getty Images

Time for a bite to eat - join us at 14:00 for more in-depth elucidation from Mr Aiken.

Looking at the regulations that were put in place to add digression to the Great Britain RHI scheme, Mr Aiken says they are "not straightforward" and he's glad he doesn't have to explain how they work.

"Even qualified lawyers will not find it an easy read," he tells the inquiry.

Image source, Getty Images

But he says that "in theory" DETI officials should've been reading these documents when considering changes to the Northern Ireland scheme.

The inquiry, he adds, will have to consider whether they did, and to what extent they "endeavored and did get to grips" with the details.

There appears to have been an assertion at DETI that the RHI scheme "runs itself", says inquiry chair Sir Patrick Coghlin.

Image source, Getty Images

Mr Aiken says that the inquiry will have to consider what the civil servants who had been working on the initiative "may or may not have been saying" to their bosses whenever the went to leave the department.

"The reality of life is that, if you go in to your boss and say 'This scheme is a total disaster, it's about to blow up, but if you don't mind I'd like to leave,' you might get a rather different answer than [if you say] 'This is absolutely no problem at all'."

As part of its response to the public consultation on its proposed changes to the Great Britain RHI scheme, DECC outlined why it was introducing digression as a way of protecting the budget for the initiative.

It said that digression was "an improvement" on its existing measure for the scheme.

Image source, Getty Images

DECC acknowledged that the application rate to the scheme had been "relatively steady and the budget is not likely to be breached" but it said it was "nevertheless essential that the government takes steps to control spend".

Mr Aiken questions whether anyone at DETI actually read that consultation response document and what they did if they had done so and seen that crucial line.

In February 2012, the Department of Energy and Climate Change (DECC) published its response to the public consultation process on its proposals for the Great Britain RHI scheme.

Mr Aiken emphasises that the information was publicly-available and could have been seen by DETI.

Image source, RHI Inquiry

DECC proposed the addition of a digression mechanism that would see "tariffs to new entrants being gradually reduced if the technology supported under the RHI is greater than forecast".

That uptake was to be monitored on a quarterly basis, with the information published on the department's website, and the statement also also announced an early tariff review.

It appears that neither Mrs Foster nor her adviser Mr Crawford made changes to the draft letter prepared by civil servants to send to banks about the RHI scheme, says Mr Aiken.

Inquiry chair Sir Patrick Coghlin remembers that there was "a fair bit" of media coverage in late-2016 about the letter, but he asks what the speculation was about.

Image source, Getty Images

Mr Aiken tells him that there was a suggestion that it was part of a "conspiracy theory" that the scheme was a "vehicle for getting more money than you should be getting and this was an encouragement to the banks to help people get into the scheme to get that money".

He says the press "may have seen the letter" but may not have had access to the material that shows how the correspondence came about.