An agreed overdraft is cheaper than a credit card. An unagreed overdraft probably isn't, but I don't know that for sure,
Mind you, I'm a bit confused at why you HAVE to pay �50 a month on such a small credit card balance. I have a similar balance, and it's only through my choice that I pay what I do. If it was down to them, the amount I have to pay is a lot less.
I personally would keep the credit card, reduce the balance as quickly as I could, and then get rid