Hey FanTom,
Firstly, excellent choice in regards to choosing Sheffield to study Japanese. I'm just going into my second year there on the Japanese Studies Degree.
Secondly, Don't worry about your GCSE results whatsoever. You'd be surprised at how significant they are past getting into 6th Form College. The main thing is - like others have said but I feel like repeating for the hell of it - the personal statement. Due to it being Ab Initio, you really need to sell yourself with WHY you want to study Japanese (and not just the language! Its Japanese Studies, they like to boast how its not a language degree when in reality it so is!)
As long as you get the entry requirements (and dont even worry about those too much, I dropped a grade but still got in and doing very well) but you really need to make sure it is what you want to do. Unfortunately the course has a huge drop out rate and mainly because people just dont know what they're getting themselves into. I've made posts about Japanese at Sheffield in a few other threads in this Forum and under the Sheffield Uni forum so take a gander at those if you want a bit more info about the course etc.
If you have any questions about Japanese, Personal Statement for Japanese, or Sheffield or anything infact, dont hestitate to ask!
Regards
Jason