If you simply want to fit it, as something to look, then it's probably a minor mod. If it's on a permit, the PFA'll do it for free, if it's on a CofA it'll cost you £62 through your CAA regional office. They'll probably insist on a suitable placard being fitted saying that it's not a primary flight instrument.
Thanks, Genghis. It's on a CofA. What's the best way to approach the CAA regional office? I was planning to send them the design drawings and FAA Form 337 documentation and tell them what I hoped to do. Is this the best tack to take with them?
It will be there for information only. I certainly don't plan the whole rigmarole.
QDM