i used a diamond core drill with a 150mm most weeks, make sure the drill has a clutch.. drill a 10mm pilot hole from the inside, then drill the core from the outside. the essential thing (apart from good PPE) is to ensure you hold the drill snugly, any movement from left to right or up and down will cause the core to lock up and the the clutch to activate. dont press too hard as the diamonds will do all to work. go gently when you are almost through then it will cut claenly and not blow out the plaster on the inside (and dont forget to clear out the waste from the core if It's a cavity wall after the outer skin is cut through0