Try Custom Colour Anodising Ltd, Barnoldswick, Lancashire. 01282 816852, or google it for more info.
They did a frame and a load of bits- chainwheels, nipples, hubs etc. for a custom build mini for my son. They did a lovely job at a good price and were nice chaps to deal with. They had a stack of old school parts waiting to be done from other radsters, so they should know what you're talking about as well.