You have dug yourself a bit of a hole with the DLA thing. DLA is as valid a form of income as the Incap and Income Support. Indeed if you were applying for a mortgage then the DLA would be a valid form of income for most banks...but the point that the bank is getting at is it is money being paid into your account and therefore it is income that you can use to spend on what ever you wish...no matter what you spend it on it is still £££, pounds, wonga, dosh into your account to spend.
If you are writing back to the bank then you are going to have to address the DLA income issue. I would suggest that you list what ever may be classed as non essential expenses and justify them as essential living costs. If this fails with the bank ask them to issue a final response regarding the matter and then contact the ombudsman.
There is also a new government e-petition regarding bank charges. I have posted the Moneysupermarket link below for you to follow and read and would ask that you may sign the petition.
http://www.moneysupermarket.com/community/forums/t/bank-charges-new-epetition-48604.aspx