I don't think unis pay a great deal of notice to GCSE grades. Or AS grades, for that matter. My brother just got offers for Geography from all 5 places he applied to (all good unis on a similar level to Notts) with a pretty poor academic record. I know that's not totally a perfect analogy to yours, but I really think it's your A level predicted grades that are the most important, and if you have them, then you're in with a very good chance. I must say that I don't know how popular music is at Notts, but it's a good course, you've chosen well. I don't do it myself, but I'm in Blowsoc and have music student friends who really like it. There seems to be a very good sense of community amongst the music students that I wish we had for Philosophy and English.
From what I've said I don't actually seem to have many links to your situation at all! But thought I'd add a voice of support from a Notts student because no-one else seems to want to: previous academic performance, especially at GCSE unless you failed most of them, doesn't seem to be hugely important. It's predicted A levels. Good luck!