Mr John first became aware in mid-2014 of a "looseness" in the regulations of the RHI scheme that allowed claimants to earn more money from the initiative than had been intended.

Inquiry chair Sir Patrick Coghlin to know when Mr John became aware that, unlike the GB initiative, there were no cost controls in place in the Northern Ireland RHI scheme.

Image source, Getty Images

There were two main forms of cost control designed to prevent subsidy payments running out of control - tiering and degression.

Mr John says Ofgem had a discussion with DETI officials in April 2014 around degression, and in May of that year Ofgem's Dr Edmund Ward discussed tiering with DETI official Peter Hutchinson.

Communications and data-sharing between DETI and Ofgem were far from ideal, as several witness have told the inquiry.

Mr Aiken puts it to Mr John that it took too long to sort out the transfer of information for the RHI scheme.

Image source, RHI Inquiry

The witness agrees that "things could have been quicker - so there were delays".

Ofgem was sharing information with DETI on a monthly basis and there was ongoing dialogue regarding the sharing of personal data, he says.

Asked by the inquiry to identify the key problems or difficulties with the RHI scheme, Mr John has made five observations in his witness statement. They are:

• Budget management and a lack of cost controls
• Increased applications in the lead-up to suspension of the scheme
• Agreement of the data sharing protocol between DETI and Ofgem
• DETI's obligation to publish data about the scheme
• Regulation amendment in December 2014

Regular followers will notice that those points have been identified by other witnesses in the inquiry and they'll be closely examined during the course of Mr John's evidence today.

Gareth John was the associate director for the RHI scheme at the initiative's administrator Ofgem’s from January 2014.

That meant he was - and still is - responsible for the operation and delivery of both the Great Britain and Northern Ireland schemes.

Image source, RHI Inquiry

You can read his witness statement to the inquiry here, external.

Inquiry junior counsel Joseph Aiken is asking the questions today.

BBC News NI

The top auditor at the department in charge of the RHI scheme was thanked for saying she accepts responsibility for a failure to spot flaws in the scheme.

Elaine Dolan told the inquiry that as the head of internal audit at Stormont's Department of Enterprise, Trade and Investment (DETI) the buck stopped with her after an audit in 2014 failed to pick up governance issues in the department.

Image source, RHI Inquiry

As Ms Dolan concluded her evidence, inquiry panel member Dame Una O'Brien thanked her for accepting responsibility for failures of the audit process.

She said her "frank" acknowledgement was an example of words heard "too infrequently so far in this inquiry".

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, 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.

Image source, AFP

Welcome to Thursday's live coverage of the Renewable Heat Incentive Inquiry.

It may be sunny outside but we're deep inside Stormont's Parliament Building for the important business in the old Senate chamber.

We haven't heard from the scheme's administrator Ofgem for a while and today top official Gareth John will be answering questions, with proceedings starting shortly.