Well, still a bit of a mystery. I have downgraded and re-upgraded to v27.1 and back to v29.1 and have checked all the connections on the unit, but still have the beige sky.
The monitor seems to be receiving red, green and blue as I can see all three colours in one place or another. The picture actually looks fine except for the sky so I probably won't try replacing the monitor because of the cost. I might try and mess around with Navtweak and see if I can change it that way. I'd welcome any other comments.