There have been posts off and on this year from people but I am afraid its one of those things - you will need to resign.
IVA started March 2011, Completed March 2016 and certificate issued 11 days after final payment. It was not always easy but then some of the best decisions aren't.
Your IP must not have known about your position prior to the IVA being approved otherwise they would have warned you and as far as I am aware there is no way round this. You can try and speak to the Charity Commission but I have only ever known clients continue on as volunteers without holding a trustee position.