Now, to the first of the closing statements - Jason Beer QC (below) is representing Ofgem.

It has already provided the inquiry with a detailed submission in writing, which you can find here, external.

Mr Beer says he'll use this time to highlight some of the key aspects of that closing submission.

Image source, RHI Inquiry

He'll address three main issues. They are:

• The failings Ofgem has admitted to the inquiry
• The main causes of the RHI scheme's overspend
• The work Ofgem is doing to improve in the administration of the scheme

But he won't be going into any comment on the information Joseph Aiken has revealed about Ofgem's handling of its investigation into ex-DUP adviser Stephen Brimstone because it needs "careful reflection".

Some information hot off the inquiry's presses...

It had previously been informed that Land and Property Services (LPS) had assessed the outbuilding (below) housing former DUP adviser Stephen Brimstone's RHI-registered biomass boiler and classified it as an agricultural building.

Image source, RHI Inquiry

However, on 22 November, LPS revised its valuation and reclassified the building as a domestic store.

"Ofgem are considering that further information in the context of the application eligibility under the RHI scheme," says inquiry barrister Joseph Aiken.

There is disagreement between senior Ofgem officials about who ultimately decided that Stephen Brimstone had complied with the rules when he applied to join the RHI scheme.

Teri Clifton and Dr Edmund Ward both say that the administrator's senior compliance manager Mark George gave the then DUP adviser the rubber stamp.

Image source, Getty Images

But Mr George disagrees - in his statement he says it was Ms Clifton, Dr Ward and other senior bosses who made the call.

He says his compliance team assessed the evidence and presented its conclusion to the management team - the management team was then asked what action should be taken.

Ofgem lawyer John Jackson, who advised against providing the PSNI with all of the information the administrator held about Stephen Brimstone, has told the inquiry there is a culture of "complete openness and transparency" at the organisation when it comes to helping police.

Explaining why he gave the advice he did, he says he didn't believe the documents that Ofgem's anti-fraud official Samantha Turnbull wanted to share with the PSNI were "critical" to its investigation.

"There was nothing in the documents, or the conclusions that they reached, that would've assisted the police in a criminal prosecution of Stephen Brimstone," says Mr Jackson in his witness statement to the inquiry.

The documents "appeared to confirm that no offence of fraud had been committed" in Mr Brimstone's RHI application, he adds.

He says Ofgem's quandary is that although it might have material that might help the police, it cannot lawfully release it or confirm it even exists unless it receives an "appropriately-worded request" for it.

But inquiry barrister Joseph Aiken says it's "difficult" to look at the Data Protection Act as having been intended to "create some sort of guessing game... for law enforcement agencies trying to prevent or detect crime".

There are concerns about the level of assistance Ofgem gave to the Police Service of Northern Ireland (PSNI) in an investigation into Stephen Brimstone's place on the RHI scheme, says inquiry barrister Joseph Aiken.

Detectives from the PSNI opened a fraud inquiry into the then DUP adviser in October 2016 after an anonymous allegation about Mr Brimstone had been passed to them by the Traditional Unionist Voice leader Jim Allister, who had originally received it.

The PSNI approached Ofgem for information relating to Mr Brimstone's RHI application.

By that stage, Ofgem had conducted its own audit of Mr Brimstone's RHI boiler and had written to him to confirm that he had joined the scheme legitimately.

Image source, Pacemaker

Among the documents Ofgem held relating to Mr Brimstone was an audit report from June 2016, which one official at the administrator's anti-fraud team - Samantha Turnbull - was keen to share with the police because she believed that without it they "might not get the full picture".

She asked Ofgem lawyer John Jackson about what she could pass to the PSNI - he said the Data Protection Act constrained Ofgem to only provide some details.

He said that while it "may not have seemed right" to withhold some information and while they should "strive to assist the police" they should only do so when they are "convinced" that detectives "have good reasons to be investigating" Mr Brimstone.

The PSNI ultimately determined that no crime had been committed.

After it received the anonymous fraud allegation against Stephen Brimstone about his RHI boiler, Ofgem did not pass the information to the Stormont department that was running the scheme.

Ofgem took the decision not to inform the Department for the Economy (DfE) - formerly the Department of Enterprise, Trade and Investment (DETI) - about the May 2016 allegation until "concrete evidence" to support it had been obtained.

Image source, Getty Images

Teri Clifton, of the administrator's RHI management team, told the inquiry that it would "not have been standard practice" for Ofgem to disclose details that "would allow identification" to DETI while an investigation was being carried out.

As a result, DETI did not become aware of the allegation until another one was made against Mr Brimstone in October 2016.

The investigation into Stephen Brimstone's RHI application was described by Ofgem as a "special case", says Joseph Aiken.

When the administrator was asked by the inquiry what exactly that meant, it explained that it reflected the "political sensitivity of Mr Brimstone's position" as a DUP adviser "coupled with media speculation surrounding the scheme".

Those were factors that meant the case "merited extra diligence and greater consideration from senior management" at Ofgem, it added.

In May 2016, Ofgem received an anonymous allegation of fraud against Mr Brimstone - who at the time was advising the then first minister Arlene Foster - regarding his use of the RHI scheme.

Internal emails show that senior bosses at Ofgem immediately noticed the significance of what was being alleged, with one writing: "Wowser."

The authority ultimately that determined Mr Brimstone had acted within the rules.

Before the closing statements begin, inquiry barrister Joseph Aiken draws attention to a significant issue of interest for the inquiry.

One former DUP adviser is a claimant on the RHI scheme and 12 new evidence statements relating to an investigation by scheme administrator Ofgem into his application have been received.

Image source, RHI Inquiry

Stephen Brimstone (above) used the RHI scheme to install a biomass boiler that heated a large shed and his home, replacing an old heating system, in order to be eligible for the lucrative subsidy that the scheme offered.

Mr Brimstone gave oral evidence to the inquiry in September - and the investigation statements run to over 1,000 pages.

Mr Aiken explains that Mr Brimstone's application - being of mixed-use or domestic/non-domestic - "is an example of an area that has caused considerable controversy".

Rather than holding these sessions in the grand Senate chamber at Stormont, the inquiry has moved to the much smaller, much less attractive Committee Room 29.

The inquiry's junior counsel Joseph Aiken opens this morning's proceedings, explaining that over the next three days the inquiry will hear oral closing remarks from the main participants in the scheme.

Image source, RHI Inquiry

In recent weeks many of those participants have made detailed closing written submissions to the inquiry.

They have already been published on its website and you can find them all here, external.

For a short summary of what they contain and what you can expect to hear over the coming days, you can read this report from our correspondent Conor Macauley.

BBC News NI

An independent inquiry into the RHI scandal was established in January last year by the then finance minister Máirtín Ó Muilleoir.

He ordered it in the wake of the huge public concern at what was then a major political crisis surrounding the scheme.

The RHI Inquiry began in November last year and Sir Patrick Coghlin (below), a retired Court of Appeal judge, is its chair and has been given full control over how it operates.

Image source, Pacemaker

It is looking at:

• the design and introduction of the RHI scheme
• the scheme's initial operation, administration, promotion and supervision
• the introduction of revised subsidies and a usage cap for new scheme claimants in autumn 2015
• the scheme's closure in February 2016

For more information on the RHI Inquiry, you can read our handy Q&A.

When the scale of the overspend emerged, public and political concern rocketed.

As the minister in charge of the Stormont department that set up the RHI scheme, the Democratic Unionist Party (DUP) leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.

Image source, PA

She resisted and Sinn Féin's Martin McGuinness then quit as deputy first minister in protest at the DUP's handling of what had by then become a full-blown political crisis.

That move brought about the collapse of the Northern Ireland Executive. Now, almost two years on from that, Northern Ireland remains without a devolved administration.

You can find much more detail on the RHI scheme in our need-to-know guide.

The budget of the RHI scheme ran out of control because of critical flaws in the way the initiative was set up.

Claimants could effectively earn more money the more fuel they burned because the subsidies on offer for renewable fuels were far greater than the cost of the fuels themselves.

At one point the estimate for the overspend was set at £700m if permanent cost controls weren't introduced - temporary cuts have since pulled the budget back on track for now.

Whatever the scale of the bill, it will have to be picked up by the Northern Ireland taxpayer.

The Renewable Heat Incentive scheme - or RHI for short - came to the fore of the Northern Ireland public's knowledge in late-2016.

The fallout from the scandal attached to it is still being felt in the region's politics today.

Image source, Getty Images

The scheme was set up by the Northern Ireland Executive in 2012 as a way of encouraging people to switch from using fossil fuels to renewable sources for generating their heat.

Those who signed up were offered financial incentives to buy new heating systems and the fuel to run them.

Just when you thought it had fallen completely off the news agenda, the Renewable Heat Incentive (RHI) Inquiry returns for a final fling!

After 111 days of evidence between November last year and the end of October this year, there is one last hurrah in the long and complex cash-for-ash inquiry saga.

Across the next three days at Stormont's Parliament Buildings, many of those who have been under the spotlight in this investigation will have a chance for a closing say on their role in the RHI scheme and its fallout.

Legal representatives will deliver closing statements on behalf of those key players and we'll bring you all the action on our live stream and out text commentary.

It starts shortly so stick with us and we'll keep you updated...