Thanks, I was beginning to think I was I on a wild goose chase as even with the inputs shorted to ground there was no difference in the on screen noise whatsoever so it was not an airborne noise or open input noise at all. I suspect that it is present on the mains supply because we use so many smps units in the modern home these days, phone chargers, LED drivers, even monitors etc use them so it is possibly coming via the mains and all I can do is tour the house pulling the plugs on as many items I can think of to see if it has any effect.
Once you know how to get into it, its a doodle, just 2 screws and is very cleverly put together internally as well, neat. I love it already.