They are claiming £5100 ...so far. Other claims might yet be lodged.
During an IVA you owe the balance of the full original debt, plus fees and possible statutory interest ... not just the scheduled payments. So, with the figures you have given, with two years interest, that figure would be around £13k less the £5550 paid, so £7450 left to pay. At you present repayment level that would take another 19 months, plus a few to cover the further interest accrued during that time --- for the hell of it, say you will hit that full repayment figure in around 2 more years, so the IVA would end a year early.
If you want to pay it off now, with a full and final offer, this would be based on remaining payments --- so 36 x 385 = £13,860. But, of course, this would take you well over the anticipated full repayment figure --- SO .... you need to base your offer on that, which is £7450.
You are saving them 2 years of costly messing about, (2, instead of 3, because you will finish early anyway, all things being equal.) so most people would reduce an offer slightly to reflect this saving, as well as the 'bird in the hand' factor. Most IVA providers would probably entertain an offer of around £6k to put to creditors. If you are with Creditfix they are likely to push for more.
Does that all make sense ? Sort of ??
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014