At this stage you will have to factor in those extra 12 months, unless you can categorically prove that the clause will not be triggered.
So your creditors are looking at getting at least a further £10,500 from you. Shot in the dark, really, as to amounts to offer -- I have seen low offers accepted and high ones refused. In your shoes, were it my IVA, I would be thinking around £8k as an opener.
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