Just thought I'd let you all know that I [i]think[/i] I sorted it out myself, without having to leave it with a tech. I don't want to speak too soon though!
I decided to open it up to have a look myself - it's well beyond warranty so there was no perceivable harm in doing so. I couldn't see anything obviously wrong with it (i.e. loose parts or disconnected wires), although it was a bit intimidating to be confronted by two circuit boards! I did, however, notice a second trim pot (aside from gain) that I'd either forgotten or not known about at all. To my ears, this trim pot controls a sort of noise gate - is this right?
Anyway, I found I could negate the problem detailed in my initial post by adjusting the gain and gate trims, combined with finding the optimum 'trigger' setting. As it turns out, I didn't need to have the trigger fader quite as high as I previously thought.
Cheers for all the help and advice - I've actually asked for help with this issue before, but gave up in that instance. Hopefully I've found the solution this time - I'm glad I've not had to spend any money on it at least!
Huw