In reply to caspar kennerdale:
Hi Caspar....you could probably get away with a gri-gri (i have used one with 9.2mm & 8.9mm ropes) but the best option would be an ATC-xp ie the one with the teeth. The guide version would also be good if you're planning on doing multipitch at some stage. To be honest all standard belay devices should cope as they are able to handle double ropes down to 8mm...if you're worried then perhaps add an extra crab for more friction when lowering.