Hi @japitts.
Thanks for all your replies. Watching the England game tonight on ITV1, there were a few brief instances of pixelation in the pre-match build-up but it's been fine since then. Like I mentioned previously, the local BBC1 (SD) channel and BBC red button channel (also SD) breaks-up as far as I remember, but we rarely watch those channels.
The example videos were from BBC2 (HD), and BBC1 (HD) which was late-night & broadcasting BBC Three.
The first engineer visit checked all our cabling to our 360 box and checked signal strength etc. No problem found.
Second engineer call out, again checked cabling, signal strength, gave us a new HDMI, checked the outside junction box, and checked the Virgin box in the road. He claimed to have found a fault in the box in the road and changed some connectors in it.
However, this has unfortunately had no effect on this annoyingly intermittent problem.
As @roy247 said, maybe our last option is to change our 360 box to see if that's the cause of these issues? Is our 360 box likely to be a "refurb" that someone else may have been using that expressed this very same problem? I naively though new customers received new equipment with their install?
Thanks