Well, after much to-ing and fro-ing I took it back for a refund today - didn't fancy sending it away as it was flawed when I got it! Interestingly the lady at John Lewis said they can only accept a refund if Apple agree but when she went to put it on the system it said "Apple no longer require a case report for this item" - seems the QC issues must be quite common!
Sadly it seems they're thin on the ground at the moment, nowhere near has any stock - I was just getting into Garage Band too, I had it hooked up to a serious system at my desk this morning and it was pretty impressive to see the rock musicians in the office go to town on it. I ended up with a damn fine rendition of Metallica's "Nothing Else Matters" saved to it, despite the drummer and bass player complaining it doesn't do 3/3 time, only 4/4...