I still haven't been able to get it to finish doing a factory reset ("searching for satellite signal" message just stays on screen) but I've done a manual re-tune and it seems to have found the satellite signal and all the channels. It says there is only one cable connected (there are definitely 2 connected from the LNB) so I can only record one channel and will have to get an engineer in to tell me why I suddenly cannot use both tuners - it all seems like an exceptionally unlikely coincidence if the cable has acquired a problem at exactly the same moment as me trying to do a factory reset! BTW, we did have some strong winds a couple of days ago, but everything had been fine since and the signal strength is at around 85% (which is fairly normal).
Looked at diagnostics menu, this is what I found:
Tuner 1: 10773, H, 22000, 5/6 strength 80% quality 50%
Tuner 2: 10773, H, 22000, 5/6 strength 80% quality 50%
should they be different or identical?