Thanks Mike.
I suspect that my problems lie elsewhere - I've just connected another tv (an older one that's been working ok) to the Tivo box, and I get the same problem ie. no signal.
The reason I bought a new tv was because during the storms the other week, my tv was hit by lightening and it stopped working. I suspect that the Tivo box was also affected. I guess I'll need an engineer to sort out the problems.
Apologies for any inconvenience.