The Department for the Economy - formerly DETI - has told the inquiry it hasn't been able to establish if there was any formal system for monitoring the price levels of biomass fuel before 2015.

Ms Hepper says the department had access to fuel price information from an industry source - the Sutherland Tables.

Mr Scoffield wants to know how it was missed that the fuel price was significantly below the level of the subsidy on offer through the RHI scheme - one of the key flaws that led to the massive overspend.

RHI Inquiry Copyright: RHI Inquiry

The witness says the understanding was that, while the fuel price was important, it was not a surprise that it was below the tariff level "because you had to add in all these other factors to it".

Inquiry chair Sir Patrick Coghlin notes that despite "all these lurid leaflets and the fact that lots of people knew outside, what that meant was completely lost on the department".

Ms Hepper confirms that was because of the officials' understanding of how the tariff was put together.