final issue sorted – the “blue” power was getting a spike from the gas boiler and thermostats, these things arc across the contacts, and the boiler has a device to deliberately create a high voltage arc (and thus light up the gas). A spark contains radio frequencies (they were once used for transmitting but this was abandoned due to interference), and the mains filters and protective earth in the boiler radiate all this back into the wiring.

So now the sound equipment too is on the “red” power. I was concerned at first whether the UPS would handle two computers (a small desktop and a laptop) and all the audio kit but modern stuff doesn’t use that much power. I can even turn the mains off at the incomer and the tunes still stay on (well for about 20 minutes maybe before the batteries run down).

Thinking that it wouldn’t take too many more batteries and solar panels to put all the AV kit off grid 😉 – clean sound and better for the environment..