petercornell thanks Peter, I'll definitely check it out.
I replaced the wire from the selector switch to the inverter with a piece of my shiny new shielded wire and that had all but sorted it out. The plexi channel is now perfect right the way up to full gain and the black face channel has a very mild version of it right at the top, which I expect I'll be able to handle by changing its wire to the switch to shielded. I'm guessing that this was acting as an antenna for the the rf you mentioned.
Incidentally, I do have a scope but since this switch was the only change I wanted to try this first.
While I was busy I also installed some caps to brighten up the single channel on the plexi and it worked well... But not quite well enough. I think I've decided to scrap that channel from the amp and reduce the complexity slightly. It doesn't do clean as well as the black face and I can just about replicate the gain with the parallel channel at low gain. That will free up a terminal on my switch that I can use to switch the input ground, which should help with the noise on the plexi channel.
