Wow! That was quick
Sorry if I caused the confusion in my opening post.
When I first connected the STB to the new TV, I forgot about the changes that could be made in the STB. Therefore, it was originally connected with 'composite', 'mono' and '4:3' as the STB settings - and it didn't work then, either
It was after half an hour of faffing about that I remembered the STB settings could be changed - so unless the on-screen options are giving me choices that the hardware/software does not actually support...?