Thanks for all the suggestions chaps.
I know it is definately something on the seatalk network that is causing the interference, as the instruments are all powered from the same switch on the main panel, and when this is off, the interference disappears.
The general area is around the flybridge autopilot head unit, as if I wave my hand under the dash in it's visicinity, the noise stops as I guess I am effectively blocking the RF. It does also happen on the handheld too.
All the NMEA/Seatalk cables are new to the plotters, but all the existing instruments were already in place using standard "Autohelm" cables. Another reason I suspect the autopilo head unit is that it the later ST7001 model, which has had it's Seatalk cable butchered a bit to connect into the older Seatalk system.
I hadn't thought of the radar, so I shall play some more at the weekend to try to isolate it.
ps, David, that was our fourth visit to Shotley. We were there last year, and twice in May as a stop-over en route to and from St Kats.