Well I picked the amp up yesterday, and the biasing is now working fine which is good! But the volume wobbling was still there :?
At the suggestion of Mrs Twinfan, I've just spoken to Marshall tech support and the actual guy who repaired it. How cool am I? :lol:
The bias problem was a resistor that had blown, caused by me putting some cr@ppy valves in (whoops!). The volume wobbling *may* be that I've got the power control on a crossover point, and moving it slightly up or down will cure it. It's not a liner 'pot' as such, but a digital control that might be between triggers. I need to get it happening again and raise or lower the power setting and see if it goes away. Something to try over the weekend.
So I've got to say top marks to Marshall for the customer service, excellent so far!