That £83 k quoted by Lisa includes statutory interest, which might be waived, or even already excluded in the agreement (it was in mine).
So my calcs would be ( and these are back of a fag packet calculations) --- to settle the whole thing:
Original debt £58,000
Fees (generally around 15%, so £8,700, plus Nominees fee and disbursements, usually around £2,000
At the three year mark, considering the reducing balance, statutory interest would come in at around £8,000
Total to settle = £76,700
At that three year mark (gratuity point) you will have paid in the best part of £45,820 (by your calcs) --- leaving £30,880 to clear.
At this point it can go one of three ways:
Either your IP will let you keep the gratuity and you carry on with the next 2 years payments --- difficult on a pension.
Or they take the gratuity and use it to hit that settlement figure and close the IVA early.
Or .... they let you keep the gratuity and you propose a Full and Final Settlement. This is a different animal to a settlement and is an offer to "pay up" the remaining agreed payments in a lump sum to close the IVA early --- in your case, at that stage, the creditors would be expecting a further 24 payments from you at £900 a month, so £21,600. This could be less if a review, based on your post service income brings the agreed payment down. But, on the £21,600, you could round it down to, say, £18k.
Clear as mud !?
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014