If you are unable to reach an agreement with your examiner you can take it higher. At the end of the day, if you cannot agree they have to either drop it or take it to court. I have to say that it rarely get taken to court so will probably be dropped. Stick to your guns, you are doing the right thing if you know you cannot afford these repayments.
thanks everybody for the advice and suggestions - i've used much of it in my reply to the examiner, who (apart from the unrealistic size of my repayment schedule) i gotta say was a nice, relaxed guy at interview who went out of his way to make me feel comfortable in what was a difficult and unusual situation for me.