Thank you for getting in touch. The problem appears to have gone away but it happened in very strange cricumstances. I contacted Customer Services by phone to report the problem, explaining that an expert on the forum reckoned we needed a replacement Tivo. He did the usual tests, reloaded the hub etc., then he asked me to check something on the TV. I switched it on, (the Tivo box was already on), and discovered that the Tivo box was going through some updating or resetting process. The call centre man said he had not initiated it. Then we got cut off!
When the Tivo box came back up, I had to re-enter the password I had previously set, and I had lost everything from the My Favourites list. It is as if the box had reset itself back to factory settings. I even had to select the Virgin network to connect it to the box.
After that, we have had NO problems with new recordings. So, in short, yes the problem has been resolved, but I don't know why or how.
Thank you for the offer of getting directly involved. If the problem returns I would like to contact you, as you suggest. I don't think the call centre man could really understand the problem I was describing and it was clearly not covered in his script.