Firstly you must specify it is an offer and not a settlement. Secondly, do you have an equity release clause? This may make a difference to the amount you have to pay if the 12 month extension applies.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
Assuming that you are not homeowners the creditors are expecting a further £3.6k from you ( nearly £6K if you are a homeowner with more than £5k equity from your share). It is difficult to gauge an offer amount without reference to the full picture, but, I would guess at something like £3k (or £4.5k if equity is involved). I have seen lower offers accepted, but I have also seen very high ones refused.
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