They are right to keep the PPI refund as long as the total paid in to the IVA, from all sources, does not exceed the full original debt plus fees and possible statutory interest. Any amounts over this are yours.
Regarding returning the interest element to you --- this is a very moveable feast. They are perfectly entitled to keep this within the IVA and are not, in any way, obliged to pay it to you, unless you formally agreed this with them. Aperture, along with a minority of other firms decided to offer to return the interest element to clients, provided they assigned PPI to Aperture ( this was the era of the 'spotty letters', before the more recent court ruling, where Grant Thorton / Aperture wrote to clients seeking a change in terms in envelopes decorated with purple spots). Basically it was a carrot in return for co-operation. However, they did not apply it across the board, nor continuously.
If you formally agreed, with Aperture, to the return to you of the interest, this should also be adhered to by Jarvis, forming, as it does, part of the contractual arrangement.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014