If you go with Diamond Cut" in Milton Keynes ....would be interested to hear how you get on.
I recently used BJV, in Hemel Hempstead, to refurb a set of diamond cut AMG Turbines....first attempt came back "yellow"...blamed on the lacquer...second attempt all four centres have corroded badly after just 2 months....so would not recommend them...!!
BJV get a few recommendations on here but I am not surprised to hear of your problems. I know someones whose had to refurb wheels again poorly done by them. I wasnt impressed with them when I took one of the TT alloys to them.