A PROBE into the “cash for ash” scandal that triggered the collapse of Northern Ireland’s power sharing executive has begun.

Sinn Fein’s Martin McGuinness resigned as deputy first minister at Stormont in January in protest at First Minister Arlene Foster’s handling of the row.

Foster’s Democratic Unionist Party is now a key Tory ally in Westminster and the sides have been unable to negotiate a new power sharing deal.

It has been more than ten months since the assembly has met and while the UK Government aims to avoid a return to direct rule, Northern Ireland Secretary James Brokenshire has now taken steps to impose a budget.

Yesterday scrutiny into the cause of the fallout began as retired judge Patrick Coghlin opened the official inquiry into the renewable heat incentive (RHI).

The scheme offered state subsidies for businesses and organisations to switch to sustainable power, but had no upper claims limit and was abused by some members. Estimates suggest the overspend could reach £700 million.

At Parliament Buildings yesterday, Coghlin said his team has already examined one million pieces of documentary evidence.

He went on: “This inquiry was established in the wake of a media and political turmoil during which strongly worded allegations and repudiations were exchanged.

“The inquiry was set up to provide a measured, objective, independent and publicly transparent investigation of the facts in accordance with the terms of the reference and that is what this inquiry shall seek to do.”

RHI subsidy tariffs were set too high and without a cap, with the scheme paying out significantly more than the price of fuel. Some applicants chose to “burn to earn”, getting free heat and making a profit in the process.

Foster, who had a central role in the establishment of the RHI scheme during her time as economy minister, is expected to give evidence in the new year.

Sinn Fein had wanted her to step aside pending the outcome of an investigation, but she declined to do so.

DUP advisers have denied any wrongdoing, including an allegation that the party held off in closing the programme to new applicants when the scale of the overspend emerged.

David Scoffield QC, the lead barrister for the inquiry, acknowledged that “precious little” of the probe’s work is “uncontentious”.

Adding that officials will not consider whether McGuinness’s resignation was justified, he went on: “It is undoubtedly part of the RHI story, that concern about what happened with this scheme reached such fever pitch that it struck at the very heart of our democratic institutions.”

Touching on allegations of “incompetence, corruption and improper patronage”, he added: “It is hard to find a taxpayer in Northern Ireland who does not have questions to ask about how a scheme intended to do so much good turned out to have an unplanned bill of such proportions as to threaten other public spending priorities, apparently well into the future.”