First Plus interest on late payments

[missetoile]

Hi, my woes with First Plus are just not ending. The latest: I was sent a letter earlier this year my newish loan holder Elderbridge saying my outstanding balance was around £10,000. Today, I spoke to them and doublechecked on the outstanding balance - and they said it was more than £24,000! When I asked for an explanation, they explained that First Plus had apparently put arrears (I've been behind on payments a few times since I took out the loan in 2005) into a separate pot "to help customer clear the arrears faster", but this pot has accrued nearly £15,000 worth of interest.
What the hell? Is this legal? What is going on? Please help!
Many thanks!