If this money wasn't taken as tax it would have increased your disposable income and been part of your regular IVA payment anyway.... so,using that logic, most IPs will ask for the refund to be paid into the IVA.
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