Okay, I think I know what I'm going to do.
Paul - I'd love a full VAG system, but unfortunately VW don't stock them any more - just the back box.
I think I'm going to go for the VAG back box with a GSF front and mid silencer (the better quality ones). I'm hoping that the exisiting downpipe will fit in okay - if not, then I'll have to look into that too (I'd rather not, as they're 89+Vat).
I would consider a supersprint 2-silencer mid section to go with the VAG backbox instead if I could find one, and the price was reasonable.
Thanks for the advice - I'm going to wait until payday, so if anyone else has any suggestions, feel free to put them forward!