Thanks to Marco for this.
Improvement to the sound using the *left* hand of the socket - tick
Move the Naim NAPSC away from all the rest of the kit - in progress.
1. Wednesday. Compared, then moved the NAPSC well away from the amp. Brilliant (bonkers...)
2. Last night. Compared again and plugged into a socket further away, same room, still on the same circuit as the hifi - another improvement to the sound.
3. Being directed by the other half just managed to stretch the mains plug into the room next door different circuit. - An improvement I want to keep - without running the cable under the door and placing it in a not good place

I boggle to think what other things can be improved with all the kit I have, but I look forward to the sound getting even better !