Hi Simon,
I had a go at taking the drive out, and have located where it is (under the plastic to the right of the trackpad), but you can’t get at it without unscrewing the screw under the “Void if Removed” sticker. As I didn’t want to void the warranty, I didn’t do that.
However, I did take it back to PC World and they looked at it, agreed it was knackered, and replaced it. You might want to try that?