Overpayment

I got convicted by the DWP in 2011 of benefit fraud. This was due to me working over a 6 year period to the tune of £9600. Works out £34 a week. Anyhow, they was taking £10 out of my fortnightly ESA benefit. This has now increased to £73.10 a fortnight. When I asked why, I was told it's because this is a fraud. I informed them they had quite happily been taking a tenner, due to hardship on medical ground's for which my doctor wrote to confirm. I explained my illnesses had not got better but worse. I spoke to a half decent chap in their collection department who advised me I now need my doctor to send proof of how my illnesses is causing hardship. I have written to them explaining this already but they want more. I included a letter from my GP explaining that I have to have a certain diet, cannot travel on public transport and need to take taxis to shop at 2am as per anxiety's around crowd's. What else can I do to get this repayment down as it is causing genuine hardship to me.