The offer should be aiming to give the creditors as near as possible to what they signed up for -- so remaining payments, including the 12 months equity release. In your case this would be 48 X 272 ... so £13056. You are saving them 4 years of messing about with small dividend payments every few months, so could round it down a little. If it were my IVA I would try for £10k, may be ready to up to £12k, but this isn't an auction scenario, they won't appreciate you upping the offer in £500 increments.
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