In November 2014, the then DETI minister's adviser Dr Andrew Crawford emailed Mr Wightman asking him to give him a call.

Mr Aiken suggests that it was "somewhat out of the blue" that the civil servant would be asked to do that without knowing what it was about and given that he'd had little contact with Dr Crawford before that.

The witness says he can't remember what the conversation would've been about.

His number was passed on as a "RHI contact" from Dr Crawford to fellow DUP ministerial adviser Stephen Brimstone, who was working at the Department for Social Development.

Image source, Getty Images

There's no trace of the national newspaper articles about the RHI scheme having being saved in the civil service record-keeping system, says Mr Aiken.

And Mr Wightman says he can't remember being shown the articles.

"Ultimately we were open to the idea of doing further legislative changes so if we did need to tighten this we could've," he says, but adds that he "cannot recall" the articles being drawn to his attention.

Mr Aiken takes us back to something that has emerged since Mr Wightman was last at the inquiry.

In November 2014 the Daily Mirror published a story about the RHI scheme that was operating in Great Britain - it contained interviews with participants who were said to be benefiting in a way that was not in the spirit of the rules.

Image source, RHI Inquiry

Mr Aiken says it was "making assertions that taxpayers' money was going up in smoke" and another article appeared in the Guardian a few months later.

Mr Wightman says he was not aware of the articles at the time.

The inquiry's junior counsel Joseph Aiken begins proceeding by reminding us that Mr Wightman (below) has already appeared before the inquiry for two full days and part of an afternoon.

He cautions Mr Wightman that the phase the inquiry is entering becomes very dense with numerous new characters emerging.

Image source, RHI Inquiry

"So if you thought it was difficult before we've got a more difficult journey coming now," he says.

"That's grand," says Mr Wightman ruefully.

BBC News NI

The inquiry was told that it will have to decide whether a statement to the Northern Ireland Assembly by DUP leader Arlene Foster at the height of the RHI debacle showed the "candour" that might have been expected.

The issue was raised by the inquiry's senior counsel David Scoffield QC.

Image source, PA

Mrs Foster addressed the assembly in December 2016 as the political row over the scandal intensified after the explosive interview given by her former party colleague Jonathan Bell to the BBC's Stephen Nolan.

In it he claimed DUP advisers had worked to keep the scheme open as he tried to close it, allowing hundreds of applicants to pile in and qualify for lucrative payments.

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.

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 the Big House - it's a cracking day up on the Stormont estate with runners and dog walkers aplenty.

We're back in the ornate Senate chamber for the first witness session of phases three and four of the Renewable Heat Incentive Inquiry.

Stuart Wightman, the former head of the Department of Enterprise, Trade and Investment's (DETI) energy efficiency branch, returns to the inquiry hotseat.

The action starts shortly so stick with us for live updates throughout the day.