Not for the first time, Sir Patrick interrupts Mr Aiken (below) to ask where his line of questioning is leading.

With a deadline set for the end of the inquiry's public hearings at the end of October, he wants to see the removal of any "virtually irrelevant material" from the proceedings and he asks: "Where are we going?"

Image source, RHI Inquiry

Mr Aiken says he wants to be fair to the witness, who claims that he was falsely named as the person who caused the delay in the introduction of cost control.

"I think you've got my point," says Sir Patrick.

A suggestion that Dr Crawford made for a change to the cost controls for the RHI scheme was a "sales pitch" to the poultry production giant Moy Park, says Sir Patrick.

Dr Crawford's plan was to increase the threshold of hours that a claimant - many of whom were poultry farmers - could run their biomass boiler for before the top rate of subsidies would be reduced.

DETI's plan was to mirror the threshold in the similar RHI scheme in Great Britain of 1,314 hours but Dr Crawford wanted that to be more than doubled to 3,000.

Image source, Getty Images

But that would've effectively rendered the rest of the cost controls plan ineffective and it was dismissed by officials at DETI.

Sir Patrick puts it to the witness that he made the proposal because there was a "special interest" from Moy Park and a desire for a higher threshold.

Dr Crawford denies that and says his reasoning was to make it easier for MLAs to agree to the changes, given that they would've been lobbied by poultry producers.

The exchange is a sharp one and Sir Patrick tells Dr Crawford that his answer shows that "you wanted to sell it to the poultry industry".

Dr Crawford explains that when looking at how to progress the changes to the RHI scheme by October 2015 his DUP adviser colleague Tim Cairns was "very nervous about the politics of it".

Inquiry chair Sir Patrick Coghlin (below) suggests that the "politics" refers to the saleability of the changes to the biomass boiler installers.

Image source, RHI Inquiry

Dr Crawford says the concern was with getting the legislation through the Northern Ireland Assembly's Enterprise Committee.

He adds that once the changes were being talked about "there would have been a lobby on it" and that Mr Cairns had ongoing difficulties with Mr Bell, his minster at the Department of Enterprise, Trade and Investment (DETI).

Dr Crawford says he would've taken a "very different approach" in giving advice about the cost controls for the RHI scheme if he'd known about how the initiative was being funded.

A key submission that civil servants had drawn up for the enterprise minister Jonathan Bell made no mention that an overspend would not be covered by Westminster and would instead have to come from Northern Ireland's block grant.

Image source, Getty Images

Dr Crawford was given that paper for his input - he says that if he'd known any overspend would've had to come from the enterprise department's budget he would've asked officials for more clarity.

"It's the starkness of the financial implications of what that is saying," he says, adding that the matter "would have to have been escalated both within the department and within the [DUP]".

A son of a farmer from Beragh in County Tyrone, Dr Andrew Crawford was the trusted adviser to Arlene Foster during much of her time as a Stormont minister and he had a significant input in parts of the RHI scheme.

Former DUP minister Jonathan Bell accused him of preventing its closure but Dr Crawford denied that.

He was also named by senior civil servant Dr Andrew McCormick in January 2016 as the adviser who exerted influence to keep it open - Dr Crawford resigned as a DUP ministerial adviser shortly after that but denied the claim and said he had "acted with complete integrity".

During his time advising Mrs Foster, he had access to all of the internal material related to the scheme and he twice sent confidential papers about it to his poultry farmer cousin, who is claimant on the initiative.

When quizzed about that at the RHI Inquiry, he insisted that it had not been his intention to give his relative an advantage but he acknowledged that "it was wrong - I shouldn't have done it".

He denied that he had a "widespread habit" of leaking government documents.

It's yet another day in the witness chair for Dr Andrew Crawford.

He'll be asked about his involvement with the scheme in the summer of 2015 when there was a rush to add cost controls to get its spiralling budget under control.

Image source, RHI Inquiry

Dr Crawford has provided several witness statements to the inquiry - you can find all of them on the inquiry's website, external.

The delay in getting started was due to the inquiry team giving Dr Crawford time to see evidence from his former DUP adviser colleague Tim Cairns that will be referred to in today's session.

The inquiry's junior counsel Joseph Aiken is posing the questions today.

Do not adjust you set - today's inquiry proceedings should've started a few minutes ago but there appears to be a delay.

Jayne McCormack
BBC News NI politics reporter

A former DUP adviser who resigned over the RHI scheme told the inquiry that he believes his party worked to "pin the blame" on him for the debacle.

Image source, Pacemaker

Dr Andrew Crawford was an adviser to Arlene Foster in Stormont's enterprise department when the scheme was set up but he quite in January 2017 after it was alleged that he had exerted influence to delay cost controls, which he denies.

He told the inquiry he was unhappy about how the DUP handled the events.

BBC News NI

It is the scandal that sent Northern Ireland's devolved government up in flames and risked leaving taxpayers hundreds of millions of pounds out of pocket.

The RHI debacle has exposed serious flaws in the region's civil service and governance systems.

BBC News NI has followed every minute of the inquiry into the scandal since it started last autumn.

Read our review of some of the major revelations that have emerged so far.

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 and what was then a developing political crisis surrounding the scheme.

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

Image source, Pacemaker

It will look 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 2015
• the scheme's closure

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, Pacemaker

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, well beyond a year-and-a-half 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 it 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... and 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 businesses 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.

Autumn has set in and the blackberries are ripe for picking on the Stormont estate.

But we're here for day 90 of the Renewable Heat Incentive Inquiry public hearings - the ornate Senate chamber of Parliament Buildings awaits more drama.

Returning to give his side of the heating scandal story is Arlene Foster's former adviser Dr Andrew Crawford.

Stay with us for a live stream of the proceedings and a text commentary throughout the day.