In my experience, store is best. If you call, you're dealing with somebody who has the huge inventory of Apple itself and might not have a manager in easy contact range. In store, stock is limited and they have immediate pressure to help you so they can move on to the next appointment.
Cheers, in which case I'm gonna try and hit the smallest store around me (London has some massive stores).
Thanks for your help. Shame the machine has been such a pain. I had a regular MBP before it and it was perfect.