So, have been inside, had a look at circuit board, nothing obvious, removed dust, squirted contact cleaner into volume pot, re-assembled.

Now have zero crackles volume control, phono output still strange. Which is what I expected to be honest but you have to try these things!

Internally it's pretty neat. All cable runs are taped down or clipped into specific slots in the case moulding. There are ferrite cores on a small number of links, and these are individually supported so they won't move about under their own weight during transit. The major audio-related electronics are inside a Faraday cage and there's also a shield under the main input/output board (which I removed to examine the PCB by the phono's).