OK, so I ordered a Forseti XL600R CDI unit from Australia - £18.20 + £10 postage so not all the World if it didn't arrive/work.
Well it did arrive after just under a month. Looks pretty good - not obviously poor quality, in fact pretty much like the Honda unit. Plugs nice and tightly straight into the loom. Started first kick! ( admittedly I had already warmed the engine using the XBR unit but has started first/second kick from cold every time since then.) First thing I noticed was the tickover was higher with the new unit, but revs out fine and noticably quicker revving from tickover.
So far work/ weather has only allowed a couple of 20 mile rides but it works fine and gives a definate improvement in pickup when cracking the throttle ( getting a 14 tooth front sprocket to make the best of it!).
So, overall, seems OK! Long term reliability is obviously a question but it looks well made so time will tell - though I have the XBR CDI is wrapped in bubblewrap under the seat just in case!
If you fancy trying it, it's under £30 - at least half the price of a 40 year old secondhand original Honda part from a broken bike.
JUST REMEMBER! If you do try one you have to disable the electric starter as if you try and use it ,it will blow the main fuse - not sure why, probably because it was designed for a dirt bike with no leccy boot. I have heard there is a way to use the unit AND have a leccy boot on a GB500 with a wiring cahnge but I'm not sure how. Maybe someone out there who understands the black art of motorcycle electrics may be able to provide an answer?