I think I have finally sorted my noise issue, it was the 3.5mm jack plug trigger lead between the AVR20 and the P49 power amp. I disconnected it and did a factory reset and it's been working perfectly all evening. I have ordered a new shielded jack plug so fingers crossed that should sort it.As mentioned, this had only been a problem with the Arcam AV860, was virtually non existent when coupled to my Lexicon LX-7 amp as using the "Ground Lift" switch pretty much eliminated it. Over the years i have changed various bits of kit and it became more noticeable when i changed from the LX-7 to the Nord amps i currently have. Short of installing a dedicated mains circuit, i'd pretty much done everything to eliminate it ...... to no avail.
As soon as i changed from the AV860 to the Trinnov Altitude 16 the hum/buzz was no more ..... so take from that what you will.