A register of risks associated with the RHI scheme, from March 2012, included "inadequate resources to deliver [the] project", which was assessed as a "high" probability of actually happening.

Among the consequences listed if that risk happened was a delay in the scheme's opening date, criticism from stakeholders, and - crucially - inadequate monitoring and auditing of the initiative.

Image source, Getty Images

"We now know that all of those consequences materialised," says Mr Lunny.

He explains that Mr Hutchinson has told the inquiry that from that point until the end of his time working on the scheme, no extra resources were diverted to the project.

Mr Thomson acknowledges that he was aware of the need for extra staff.

In February 2012, DETI civil servant Peter Hutchinson (below) - who worked on the set-up of the RHI scheme largely on his own - was given a pay bonus for his involvement with the project.

Image source, RHI Inquiry

On a form nominating him for the award, his colleague Joanne McCutcheon wrote that he deserved it because he was in a "very heavily-loaded post" and had "consistently gone the extra mile".

Mr Thomson approved the bonus and says he was aware that Mr Hutchinson was working evenings and weekends on the scheme.

In October 2010, DETI received the offer of £25m funding for a renewable heat project, which Mr Thomson describes as "significant".

That was seen as particularly attractive given that it was ring-fenced, meaning it would not be swallowed up in the Northern Ireland Executive's wider funding pot.

Image source, RHI Inquiry

Inquiry chair Sir Patrick Coghlin observes that it may have been a financially attractive offer but there were other factors to be borne in mind.

"It's an offer where you were going into with your eyes open that those who were going to deliver this project were going to badly under resourced."

Again, Mr Thomson agrees.

Sir Patrick says the evidence the inquiry has heard from the DETI officials who were working directly on the RHI scheme shows it was an "under-resourced project" and there was a "constant request" for more staff.

He wants to know why DETI decided to start its own RHI scheme "if you knew you were under-resourced" when it could've saved itself a lot of bother by joining the similar initiative that was running in Great Britain.

Mr Thomson says he remembers a discussion with Mrs Foster in 2010 in which she gave the go-ahead for a project to incentivise renewable heat use.

Sir Patrick sums it up that "the upper level of the civil service, and/or the minister" decided to run with the scheme even though they knew "the department that would be doing it was badly under-resourced".

Mr Thomson agrees.

It was "very difficult to stop things" at DETI once they had been set in motion, Mr Thomson recalls from his time there.

There was "difficulties" for people within the department who said projects couldn't be done, he adds.

Image source, RHI Inquiry

Inquiry panellist Dame Una O'Brien (above) queries if decisions were ever taken to park projects in order to divert resources to higher priority matters that couldn't be halted.

Mr Thomson says decisions of that type were taken, and management was "looking across the whole piece".

In his written witness statement, Mr Thomson has said DETI recognised that its energy division was under a particularly heavy workload and had provided some extra resources for it.

Asked whether he thought the division had the required expertise, Mr Thomson said he did "because they had a lot of good people in there".

Image source, RHI Inquiry

"Fiona (Hepper) was very good, she wasn't an energy expert" but the trouble was that "it was a division which was under a huge amount of pressure".

He said that despite drastic manpower cuts in the rest of the civil service, DETI bosses supplied extra resources to the division "because we were very concerned about it".

The RHI scheme "wasn't ignored" by DETI's top management team but nor was it at the top of the priority list, says Mr Thomson.

In forthright fashion, the inquiry chair Sir Patrick Coghlin wants to know who made the decision for it not to be a priority.

Image source, Getty Images

Mr Thomson says it was a "joint decision" by the then DETI minister Arlene Foster (above) and the department's top management team, of which he was a part.

"You had to take decisions as a professional as to what you put your attention to... RHI wasn't one of my top priorities that I was spending a lot of time on."

Mr Thomson was involved in a number of DETI's special projects, but those didn't include RHI.

The initiatives he was closely involved in included the development of Titanic Belfast in 2012, which involved a £70m public investment, and the restoration of the warship HMS Caroline as a tourist attraction.

Image source, Reuters

Asked why the RHI scheme was not seen as a priority, he explains that some projects had to be done within a certain timescale - such as Titanic, which "had to be finished and opened for the centenary" of the disaster that it was built to mark.

"I couldn't have gone to the minister and said: 'Minister, we'll make this but it'll be four weeks late," he ads. "Things like that drive priorities."

Mr Thomson says that 2013 was "the year of most pressure" in terms of senior management staffing for DETI.

He explains that there was a lot of work being done on the economy as a result of the recession and the main staff member responsible for that work was off due to a serious illness, leaving the department "short... on what was a critical area".

Image source, RHI Inquiry

He says he and the then permanent secretary David Sterling - now the head of the civil service - "tried to carve up responsibilities as best we could", with Mr Sterling agreeing to keep a "watching brief" of the department's energy division for about six months.

Mr Thomson then took formal responsibility for the energy division in October 2013.

Image source, DETI

Mr Lunny outlines the six areas he's going to be asking Mr Thomson about today:

• Resources, especially in DETI's energy division and its renewable heat branch

• Project management

• His involvement in the RHI scheme
• His monitoring of the scheme and DETI's energy division
• The review of the scheme that was supposed to happen in 2014
• Cast controls
• Staff changes and handovers

With the inquiry's junior counsel Donal Lunny doing the honours today, Mr Thomson is sworn in and takes his first few questions.

Mr Thomson was part of the top management team at DETI from January 2010 until his retirement, and he explains that most of the details of his work at DETI no longer exist.

Image source, RHI Inquiry

In preparation for his appearance at the inquiry, he asked the department if he could have access to the internal records during his time working there but was told they had been deleted.

As for his own notebooks relating to his work at DETI, he says he kept them for about a year after his retirement and then shredded them himself.

You can find his written statement on the inquiry's website, external.

BBC News Northern Ireland

A civil servant who temporarily ran the DETI team dealing with the RHI scheme said she did not enjoy her time working on it.

Image source, RHI Inquiry

Davina McCay had a seven-week spell between May and July 2014 working on the project, and told the inquiry she was relieved when her secondment was not made be permanent.

She said her role was "extremely busy" and was made all the more challenging because experienced staff had either left or were leaving the team.

BBC News Northern Ireland

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 concernand 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, Press Eye

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, more than a year 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.

The most recent estimate for the overspend was set at £700m, if permanent cost controls aren't introduced.

The massive overspend bill 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.

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.

Come in out of the cold, why don't you, and join us for Wednesday's grilling at the Renewable Heat Incentive Inquiry.

Image source, Press Eye

Today's witness is David Thomson, a senior civil servant of long standing, who retired in 2014.

Mr Thomson's responsibilities ran right across the remit of the Department for Enterprise, Trade and Investment (DETI), from energy to tourism - we'll be hearing from him shortly.