After a few hours answering questions yesterday afternoon, John Mills is back at the inquiry this morning.

He managed DETI's energy team - which was responsible for the RHI scheme - from January 2014 until May 2016, during which the scheme spiralled out of control, burst its budget and was eventually shut down.

Image source, RHI Inquiry

If you fancy a long read, you'll find Mr Mills' full written statement to the inquiry here, external.

The inquiry's senior counsel David Scoffield QC is posing the questions.

BBC News Northern Ireland

Northern Ireland's most senior civil servant denied that the department he formerly ran was a "serial offender" in terms of problems with public money.

Image source, Press Eye

It was suggested to David Sterling, who headed DETI during the set-up and initial running of the RHI scheme, that the department had not learned lessons from two previous projects that had seen money squandered.

He acknowledged that the civil service has much work to do to to regain public trust after the RHI debacle.

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), 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, 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 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.

Welcome along to what is the last day of hearings at the renewable heat Incentive Inquiry before the Easter break.

We know you'll miss us, so be sure to savour what we have for you today.

Returning to the Senate chamber here at Stormont's Parliament Buildings will be the former Department of Enterprise, Trade and Investment (DETI) energy boss John Mills - we'll hear from him first.

And a former DETI finance boss Trevor Cooper will be here in the afternoon.