I was advised by my school that my GCSE's were the bare minimum for any Oxbridge application. I got 6A*s, 4A's and a B at GCSE and was predicted A*A*AA at my A Levels in Maths, History, Chemistry and Economics. Everyone I met at interview had better GCSEs than me but I was still given an offer at Cambridge this year. My advice would be to write a killer personal statement in order to really make the fellows who would interview you want to talk to you! Also, when you apply, do so to a smaller college as the smaller colleges tend to interview a higher percentage of applicants. I don't know the stats on GCSE's for successful applicants but D grades are unlikely to go down too well, however, if you have space on your UCAS, certainly go for it. You have nothing to lose.