Permitted Work

I am in the WRAG of ESA and started permitted work (or so I thought) at the beginning of May this year. I am working just 15 hours a week and when I started I was unsure how much exactly I would be taking home as I had only been told the gross pay pro rata and did not know how to work it out. I did tell the DWP this at the time and as soon as I received a full months pay slip I sent it to them. I then received a letter saying something along the lines of it being OK and that I would continue to receive my ESA while working etc. Unfortunately I cannot find this letter anywhere and the DWP don't seem to be able to find a record of it either. Anyway, what has happened now is that it turns out I have been earning over the permitted £120 a week and although it is only over by £2.78 a week they have told me I am not entitled to ESA after all and have stopped my ESA altogether as well as demanding back all the ESA I have been paid since May. Firstly, I would not have continued working had I known, ie if they had not sent me a letter saying it was OK to work and that I would still receive benefits. Secondly, if I had been earning such a small amount over, I would have assumed that they would simply deduct that amount from the ESA they paid me, not stop me the full benefit of £102.15 a week. Has anyone else had these problems and does anyone know what can be done about it if anything or if I have to give up my job and go back to square one again?