Thanks for your response.
* No, it seems to be random:
# A scheduled TV show will not be recorded and indicated "Time missed" but nothing else is scheduled or recorded at that time.
# A scheduled TV show will be recorded but indicated "Time missed" because another episode of the same TV show was being recorded at that time - for example Wednesday broadcast of Rising Damp recorded on Monday. Of course not physically possible unless HTS is capable of time travel!
# A scheduled TV show will start to record at the correct time but will not stop recording such that everything is recorded after (until stopped by myself - not run out of disk space yet).
# A TV show not scheduled to record will be recorded.
* I haven't checked every channel. It is definitely happening with UK channels ITV 3 and CBS Action (Rising Damp and Hawaii Five-O and Star Trek)
* Time on RPi is correct. I check periodically and on reboot.
What I have done is limit access to OpenELEC/RPi to one browser, Chrome as the problem may have started since using Firefox and Chromium on other systems. I have seen similar behaviour when accessing OpenELEC with Android apps so I'm hopeful this will solve the problem. Its only been a day so far and no more Rising Damp until Monday. Many thanks for your help and suggestions.
I have tried to add time stamp and other details to the file name but it hasn't worked. I'll fiddle with those settings again, Google them - probably I've missed something obvious.