Got home and had another look. Now when I try to schedule a program at the same time as Coronation Street I get the warning telling me I have to cancel a scheduled program - both of them being Coronation Street!
Prompted by zulu17's question about channelsin the 800 range, I looked and sure enough there at 803 is another ITV HD and when I set Coronation Street to record it gets set on both Channels 103 and 803 - if I cancel it one one it also gets cancelled on the other.
Now the question is how do I get rid of channel 803?
Update: I think I may have fixed it, although I'm not quite sure how! In the Scheduled Recordings page, there were two tiles for Corrie on Friday. I deleted one and the other remained. I checked the guide and only the Corrie on 803 was set for recording. I cancelled the recording on 803, went to 103, selected it for recording and this time 803 was not scheduled! Everything seems to be back as before - touch wood!
Thanks to everyone who took the time to read and reply.