The fix was to insert a noise filter in between the amplifier and the power source. With my pathetic electrical skills this took me a wee while but it's all together now and on a quick test ride the problem appears to have been fixed - bonus!
While fluffing around I also noticed that my right headlight bulb had blown (less than 1,000km after the left one) so I think I can confirm that Phillips Xtreme power +80's last around 20,000km. Luckily I had a spare so that has gone into the bike and I've ordered a set of the new Xtreme Vision +100's from Power Bulbs which should get here in time for GC so I'll be able to take spares with me.
Now all I've got to do is to give the bike a decent clean and she's ready to rock for the GC and Four Points!