When considering whether to take on the administration of the RHI scheme, Ofgem weighed up the potential risks of doing so, and four are listed in an internal document from the time.

Among them was that timescale and cost would be affected if Stormont's Department for Enterprise, Trade and Investment (DETI) "delays making the final policy decisions or makes significant changes", which ultimately happened.

Image source, Getty Images

Another was that there may be a "failure to agree the terms that Ofgem was proposing for the work, and there did turn out to be conflict between DETI and the administrator over the costs.

A third risk was if DETI's scheme was to include extra renewable heat technologies in what could be subsidised, which indeed it did.

That prompts an observation from Sir Patrick: "Three out of four aren't bad - three out of four concerns seems to have been reasonably prescient."

In considering taking on the administration of the NI RHI scheme, Mr Harnack says that Ofgem was asking itself: "Why should Ofgem do this?

"What are the benefits for GB consumers? Because Ofgem looks after the interests of GB consumers."

Image source, RHI Inquiry

Inquiry chair Sir Patrick Coghlin wants clarification: "What benefits were you concerned about? Benefits only to GB?" he asks.

"There had to be some benefits for GB consumers for Ofgem to justify being involved. I wouldn't say it was the only thing we would be concerned about," Mr Harnack replies.

He explains that there would be savings in areas such as sharing fixed costs, such as IT.

Matthew Harnack, who was a senior official at Ofgem when the RHI scheme was established, takes the affirmation.

Like yesterday's Ofgem witness Catherine McArthur, Mr Harnack moved to Ofgem from Australia having worked for the government of New South Wales.

Image source, RHI Inquiry

He worked on new scheme development in Ofgem's E-Serve division, which delivered green energy and social schemes on behalf of the government.

He didn't work day-to-day on the RHI scheme day but dipped in when some issues with it were escalated to him, outlines the inquiry's senior counsel Mr Scoffield.

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

BBC News Northern Ireland

Mistakes that happened with the RHI scheme could be repeated, the inquiry heard.

Catherine McArthur, who drew up a feasibility study on the initiative administration costs, said that key problems included "unrealistic timeframes" and a lack of understanding of the complexity of the work to set up the scheme.

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

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

Image source, Press Eye

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, 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, 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 autumn last year... 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.

Chilly, blustery sort of a morning up here on Stormont hill, but the heat is blaring in the Senate chamber for the RHI Inquiry.

Image source, AFP

After two witnesses from the RHI scheme's administrator Ofgem appeared yesterday, Matthew Harnack - also from the organisation - will answer the questions today.

We'll have a live stream and text commentary throughout the day, starting shortly.