It was 30 odd years ago somewhere in Leeds. I'd trashed the engine up on my Bonnie and got a replacement engine. It was the T140 Jubilee which had chrome casings, the primary chanecase was missing so I got a standard ally one and had it chromed along with the fork legs which were also ally.
Try the yellow pages (electro platers), there are five places in Hull so I imagine there will be plenty in more civilized places.