Problem solved - the TV had somehow switched to hotel mode, while at the same time de-tuning itself. Not sure how that could have happened, but sorted all the same.
For reference, hotel mode is switched on/off using the service menu. To get to it, turn the TV on whilst holding down the i button on the remote.
Again for reference, and to make this more easily found in searches, the chassis number for the st 63-725/8 is cuc6330.
Ross
October 2005