It may help if you are say you are prepared to show them your statement of incoming and expenditure.
This will help prove to them that you are taking the situation serioulsy and genuinely do not have any more money to give, and that they are not being asked to take less, if you are still paing full amounts to to other creditors.
There is a good template to use here, with some goo tips and info on what sums to allow as a budget for things like clothes etc:
http://www.homeworking.com/library/debt/6.htm
It may look arduous and invasisve, but remember you are asking a business to let you alter your contract with them!