bush 6690d vertical collapse problem

[buzlight]

The TV is an integrated FreeView/OnDigital. It is a CRT model. Truth is we just bought a big plasma for the lounge and this Bush set (6690d) has suddenly decided to give us a viewing area of 50% where the area comprises a central band whose edges are slightly curved inwards. The picture is perfect in reception but annoying. I suspect that the fix could be relatively cheap and since we really like the set we would like to get it back to normal again. I think the term given to the problem is "vertical collapse".
Thanks.
Buzlight.