Last night I tried a test. Over night I left the Youview box switched off from the power supply, so it would have a cold start in the morning.
This morning when everything was powered up, I had perfect viewing, no interference or pixels. After 3.5 hours the interference returned again. Now I think I can safely assume it is the Youview box at fault.
It does sound like a bit of a strange one that the interference is intermittent. I'd like to take a look at this for you and run a few check on your YouView box. Please can you send me in your details using the "Contact The Mods" link found in my profile.
I have had an enginer visit this morning, arranged by BT Consumer Help. He found nothing obviously wrong. The building I live in has a communial antenna which is not a normal style aerial, but is a vertical single rigid pole (no idea of it's correct name). He suggested I purchase a booster as it appears the signal strength is dipping below 75%.
I will give that a try as a booster is not expensive.