By paying the settlement figure that your IP provided, you were basically agreeing to pay the balance of the debt, as it stood at that time .... unlike a full and final which would have drawn a line under it by creditors agreeing to accept the amount offered and calling it quits .... with the settlement, no one has called it quits and HMRC have put in a final claim, so the debt balance increased and that quoted amount no longer covers it.
HMRC are treated differently to other creditors, so I would advise that you have a word with the IP to see where things lay.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk