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, and the neighbours were watching both analogue and digital without problem (so not a transmitter issue).

The missing Prime analogue was the clue - seems that one ariel (VHF?) provides TV1-TV3 analogue, and the other (UHF?) provides Prime analogue and all the digital channels. After the TV Man (tm) climbed up to the roof, it was identified that something (lightning strike? high static in the air?) had fried not only the VHF+UHF combiner box, but also the distributor box in the attic which splits the ariel to the 4 points around the house. Very fortunately, the Signal booster, TV and TiVo were unaffected by this - anything that could take out the first two boxes in the chain could easily knock out the TiVo tuner as well, I'd have thought.

Cost to fix - $250, for replacement units, callout and labour. All done just in time for the evening TV shows. Might be an idea to get a surge protector in the line to protect the TiVo in the future?

Strangely, I remember no electric storm that night, although it could just be a high static in the air or something. I have the broken units for investigation but not much I can do with them anyway.