They can take all of it unless and until the debts, statutory interest (if applicable) and costs have been paid in full. Only then would any surplus be returned.
If you inherit prior to, and there are funds left as an asset of yours, or during the IVA, the inheritance will be due to the creditors up to the balance of the full original debt, plus fees and possible statutory interest. Anything over that figure will be yours.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014