I am a bit disappointed with your opening sentence, Nigel. Yes, we are a long standing and, I have to say, pretty popular IVA support forum. But, for the most part, we are only people. like you, who have gone through an IVA ourselves and share our experiences. To be fair, not many of us have gone through failure of our arrangements and so cannot comment from experience. We can only offer opinion and, yes, we often see the worst case scenario as we do not wish to lull someone into a step they might regret later by offering candy floss and unicorns.
It seems that you are at the mercy, as you put it, of a greedy debt purchaser --- they are out there and some are worse than the original creditor, but many are not. Let's face it, chasing bad debt is not the friendliest of occupations !
In my experience -- purely anecdotal -- when an IVA fails creditors are entitled to revert to the original credit agreement and recover ( or add) 'lost' interest and charges. As debt purchasers are bound to the terms agreed by the original creditor, I assume that they can do the same --- maybe I am wrong, but better to be prepared for the worse yet hope for the best.
I agree that you seem to be getting a rough deal here, but assure you that this is not always, nor indeed usually, the case with IVAs in general.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014