I've now deduced my problem isn't a signal issue. I have BT Vision and a Samsung TV, have retuned both and the BBC channels are breaking up whether they're coming through the tuner in the TV or the BT vision box. However, when the television is off and the BT vision box is recording one of these channels, the recording is fine.
The TV must be causing some interference with the new location of the BBC channels but I have no idea why or how to rectify this.
The best Digital UK could advise me was to switch everything off, wait 5 minutes, switch on again and retune... why didn't I think of that!