Engineer attended and found the RF levels too high.
It appears that when I returned an old NTL Samsung tv box from upstairs via the 'send a box and post it back to us' system the inbound RF signal then became too high. It was set when we had two Tv boxes and a WiFi by an engineer. Until I posted a comment, as above, the issue had gone undetected for some time. I must admit though, I hadn't noticed anything of any detriment to my system.
Remedy, Filter on the Tivo box. Sort out a redundant splitter and set level accordingly on both Tivo and WiFi Hub.