For what he probably hopes will be the final time, Stuart Wightman enters the Senate chamber to answer questions from the inquiry's junior counsel, Joseph Aiken.

Quick reminder about Mr Wightman's role - he headed DETI's energy efficiency branch from June 2014, meaning he had oversight of the RHI scheme.

Image source, RHI Inquiry

This is his sixth appearance at the inquiry, withhis first having been back in March.

If you want a glance over his witness statement,you'll find it on the inquiry's website, external.

BBC News NI

The risks of the RHI scheme were "significantly downplayed" by senior officials to a new Stormont as the energy scheme began to spiral out of control, the inquiry was told.

Language in a key submission to then enterprise minister Jonathan Bell in July 2015 was watered down in a series of drafts.

Image source, Pacemaker

The submission aimed to set out the problems in the scheme and the cost controls needed to address them.

But an early version, which clearly stated the projected expenditure on the scheme was twice the available budget, was amended to remove the "stark" warning.

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, 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, 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.

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 to Wednesday's live coverage of the Renewable Heat Incentive Inquiry - we'll be going over to the Senate chamber in Stormont's Parliament Buildings shortly.

This morning sees the final witness session with Stuart Wightman, who oversaw the running of the RHI scheme when he was the head of the Department of Enterprise, Trade and Investment's (DETI) energy efficiency branch.

And this afternoon we'll hear from his boss John Mills.

Stay with us for live video coverage and commentary throughout the day.