Fees, in a nutshell, will be the nominees fee, for preparing the IVA, typically around £1750, the ongoing Supervisors fee of, as you say in this case, 15% of anything you pay in. Added to this are any variation fees, for additional meetings and disbursements, typically £500 - £1,000.
So to settle an IVA in full you need to pay these fees, plus the balance of the remaining full debt. You might also be required to pay statutory interest at 8% per annum simple, calculated on the reducing daily balance of the debt.
So --- on the back of a cigarette packet guestimate on a debt of £14500 at 3 years:
Nominees fee £1750
Supervisors fee on £14,500 @ 15% is £2175
Disbursements, say, £1000
Statutory interest --- around 1160 for three years, so, £3480
Debt £14,500
Total to clear everything £ 22,905 Less £3000 already paid = £ 19905
If you throw VAT in on their fees, it will be close to their figures.
Oftentimes creditors are asked to vary the agreement by forgiving the need to pay statutory interest. They also often agree, so, if they agree this time (no guarantees) it could save you over £3k. Worth as ask.
If you cancel the DD and deal with creditors outside of the IVA they will add back in contractual interest ( far more than statutory interest ) as well as possible charges and the IP will then become another creditor after you for their money.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014