Ok, admitted defeat and got an expert in. GT satellites from Gorseinon.

First thing he said, your dipole is the wrong way round.

Tested the signal, just over 40db.

Took the dipole of, checked the connection and put it back on the correct way. Signal now 57db on C58.

Signal now strong enough to run both boxes.

FTI on both and picture quality excellent. Fingers crossed.

Many thanks again for all your help and the recommendation on the Bush box.

Glad you got it sorted. How long has the aerial been setup incorrectly? I presume it is a roof aerial?

Yes, roof aerial. For a good few years.

It has always been that way since we put it up, probably 5 years ago.

Annoying that it worked so well all that time, in a way . Wouldn't have been something I'd even dream of suggesting, but it does make me wonder that perhaps at least some of the people who have had trouble with Vestel boxes, particularly after DSO and other retune events, had an issue with their aerial installation.