Its up to the creditors what they do once an IVA fails, but it would be quite normal for them to add on interest and other charges (including what accrued during the IVA period). However, after a time some of them will probably have passed the debt to a debt collection agency to try and collect on their behalf, or have sold it to a third party. It is quite common (but not necessarily the case) that, when either of these things happen, interest and charges stop being added. If you get asked for an amount which you think is unreasonably high in relation to what the debt was originally, you could ask for a detailed breakdown - but many firms seem reluctant to provide this.
You could try for full and final settlement offers and some of the creditors may accept. It would depend in part on what information you were able to give them about the source of the money you now have available - eg if someone was being helped by family up to a certain amount and was able to say that this was the maximum available it might be more acceptable to a creditor than a simple statement that you now have the money to settle.