Well, I went with GiffGaff in the end and I took out their £12 goody bag (250 mins, unlimited texts, unlimited data) and am chuffed to bits. Initially, I had some minor issues getting the data working and had to delete all my APN's and fiddle about adding new ones, however after that, it's was good. Signal is spot on (as I would expect) and have no issues with playing music through Google Music. Even got it set to always play high quality mode too, and it works a treat.