Well, woke up in the morning and the TiVo wasn't working.
Further investigation (using the channel signal strength monitor in the TiVo menu) revealed that there was no digital signal coming in, and I could pinpiont the time rather well by tracking which suggestions had been recorded at what time. Had our TiVo died?
Analogue channels on the TV were working, apart from Prime which was missing. A visual check on the roof showed the ariels (VHF and UHF) looked fine,