Hi Anthony. The Deed of Assignment is really a belt and braces job to make dealing with PPI after the IVA simpler and to allow you to get your completion certificate faster.
Simply put: whether you sign this or not they will go after any PPI refunds, as these are considered an asset and due to be paid towards your debt to your creditors ... it will simply take much longer for you to get your completion formally finalised if you do not sign it.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014