The bloke at the paint shop was no help. He said Yorkshire Electrical green meant nothing to him, neither did Ford Mustang green (I really like the Mustang green). He could only look up RAL numbers or match a sample, which I didn't have with me. I did get the impression that his IQ would almost put him in the intellectual disability category though.
Grant offered to send me his side panel to match, but it seems a bit mad to post a side cover from the other side of the world and back again. I have a mate here (Aldo) with a green 3C that I can borrow a side cover from. That'll give me a reference base to start from and I can then decide whether to go for a different shade.