Thanks both.
Don't think I've got the time or energy to rig up the tivo to a dvd recorder even though I could, not since I lost months of my life digitising all my video tape stuff.
Apparently you get to keep the old Tivo if they are willing to swap you out, and the ability to watch pre recorded shows continues for a while, although eventually the facility will disappear especially if the box is rebooted.
Also there's no guarantee they'll even give me a new box, although I guess if i'm negotiating a new contract that may tip it in my favour.