No problem, I can do that…
The volume ‘pot’ (potentiometer) is soldered into a small PCB at the front of the amp, and the cost of a new part is £3. I fact I couldn’t economically source an exact part, but one with a longer shaft will do, we’ll just nip the top off. The new part does however have a locating pin (arrowed in red) which helps to keep it in place when it’s mounted.
Having got everything back together the scratching/static noise is still there, so I investigate further to find that the valve plate (anode) power supply isn’t at the right voltage, and the cause is a blown fuse. There appears to be no reason for the fuse to blow, so I replace it.
All working again (and the VU meter, based on the 6E2 valve, works too which I should have noticed in the first place)