Its not too late to start asking for paperwork from Barclays to justify their claim. Don't fill out the income/expenditure form and don't send any bank statements to them.(posted 8347 days ago)I think you should stop paying the £25 per month for now, because you don't actually admit liability for the debt. I'm sure that others will correct me if I'm wrong there.
Read through this website, get copies of the completion statement and all the paperwork regarding Barclays claim. You may find that they'll start threatening you with court action etc if you don't do as they ask.
You may benefit from seeing a solicitor about your situation as you have other debts apart from the mortgage shortfall.