On those figures I am guessing that your payments are about £150 a month with around 40 months left. The usual way to work out a reasonable F&F offer is to offer the amount creditors are expecting.
In your case this would be the regular payments over the remaining 40 months, so £6000. As regards to equity release, there is no guarantee that you will be able to release this 'expected £8000', so a homeowners F&F usually incorporates the only amount the creditors can actually depend on -- the 12 month extension .... in your case, a further £1800.
So they would be looking at getting, in all likelyhood, £7800 over the next 52 months. So the offer could be based on that figure, and even reduced slightly to reflect savings in admin costs over the next 4 years or so. So .... an offer of about £13k will probably see your hand being bitten off !
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