Many thanks all who have responded. I've spent hours doing test recordings and settings adjustments.
First thing to note is that timer recordings which begin while the box is on DO record and don't wipe the rec schedule. The previous failure must have been a one-off.
Biggles may be onto the answer with the 9300 'return to default' bug. After a standby, it does indeed wipe my settings for aspect ratio, skip times, Rec schedule etc. (Just switching the box to Standby is enough to reset to defaults and wipe the Rec Schedule. It doesn't require a failed timed recording.) Did they come up with an solution on the 9300?
As to the RTC, what should I conclude from the fact that the set always comes on at the programmed time, yet fails to start recording, just outputting audio? (Audio of the channel it had been tuned to, not the channel I set to record, btw).
It has clearly remembered the time - and suggests whatever wiping goes on, happens at startup rather than at switch off or the time in between. RTC exonerated???
To add to the fun, I have now found that sometimes it WILL perform a timer recording. This occurred twice after I speculatively changed the 'Power Saving In Standby' to 'On' from the default 'Off'. Sadly, it mostly doesn't work, and I can't find any pattern as to why.
Thanks again for your suggestions. I hope you can generate some more with the new info.