Hello Burantek, Frog and Chaser.
Black tape - nice and simple, thanks! The HDMI to DVi adapter is a very tight fit on the TV (and not the most elegant of solutions as it comes loose easily!). I have pushed all the cables in securely and also put a cable tidy over them, which might help to keep them from moving. The TV is on an STB bracket (so when it moves the cables move also).
I shall enqure about the scaler board. Although, if that was the case, wouldn't I have the same problem of only half a screen showing on the ordinary TV selection? It never happens when it is terrestrial TV.