I want to know how tax repayment interest is calculated by Customs & Revenue?

I have just received a tax repayment because I backdated terminal trading losses to previous years. The repayment includes tax overpaid for 2004/05; 2005/06 and 2006/07 but I have not received any interest supplement on the repayment. I appreciate that Customs & Revenue are not paying any interest on current repayments but I thought that if a repayment was being made for past years interest was payable at the rates that have applied during the period from that year up to the present time. Before I query this with the tax office I would like confirmation that my thinking is correct.