I'm no expert, but just going to bat some ideas about. Brands I remember using wishbones are Muddy Fox, Saracen, Orange and Bontrager. Of that list, I think Bonty is most likely, with the Ritchey connection. Other possible is a custom English builder. Might be worth e-mailing a pic and frame number to the likes of Dave Yates, Chas Roberts, Overburys, Argos etc to ask if it's one of theirs. It's also possible that one of them did the disc modifications and might remember what it is.