Hi all,
I thought I'd give you an update. Following your comments that you can't measure charging circuit voltage with the bike running, I connected the output of my C15 rectifier/regulator to a battery on another bike and started up the C15, it held a fairly steady 14 and a bit volts. Then when I reran this test on my old C15 battery (which is still hanging around the garage), the voltage actually dipped sharply when I revved the bike. Which all worked to confirm that the charging circuit on the C15 is working ok, and the old battery I had was done in. I took the bike for a little spin today with a new battery and it seems to be charging fine. Yay!
Looks like it was just an old battery I had which was causing the problems, but because I didn't know the charging circuit voltage normally looks bonkers on a digital multi-meter while the bikes running I went out and bought a new reg/rectifier for no reason at all!!
Anyways, thanks for your advice.