Lepsons get great reviews. Ive personally used Wicked Wheels in Crawley for three sets of wheels and their work is really good. Normally they want £55 per wheel though Ive not had them do diamond cut.
My brothers staff work there & apparently its the nuts, could of got them dome for £25 each wheel but that means travelling to them and no warranty as it would have been done out of hours rather pay the extra and have piece of mind if i have any trouble.
I have used the Wheel Specialist in Nottingham twice and even though it worked out about £60 a wheel for 18inch.
You drive in first thing in the morning,they remove wheels,tyres, blast the wheels paint and lacquer them, tyres refitted and balanced and you pick your car up later in the day all done.
I will never go anywhere else.