Ho, Hum, Well I dunno, I’ve done everything that you’ve all kindly suggested in this thread, and the noise is no longer intermittent in the MG.

1) Just to be sure, I’ve re-charged the battery in the motorhome and put the Empeg in, it’s absolutely clean up to full volume.
2) I’ve noticed that when I turn the volume up way above listening level in Mark’s dock, I can hear background noises, but not appreciably.
3) If I plug it in to the home supply it’s absolutely clean.
4) I dismantled the sled and took the top off the Empeg, every pin is connecting correctly from inside the HU, through to the back of the sled connector.
5) Where waggling the wires could stop the noise, it no longer does.
6) The battery seems to be holding its charge O.K.
7) I’ve connected the amp to a spare HU, it works fine.
8) I’ve by-passed the fitted negative wiring with 70 amp cable direct to the pin on the dock.

The only thing I’m not sure about is how to check for a fault in the alternator, as I say, it seems to be charging properly.
Any other bright ideas? I’m going to take it back down to the fitting centre on Monday, but as I’ve said, they can’t find anything obvious.
Pity I’m not taking it to Amersfoort, this year, for people to scratch their heads over. But I don’t fancy crossing Yorkshire and Holland with a loud buzzing, anyway.