I haven't got a spare box to try GS again on, but checking the basics you do select:
'Cable' as your source
'07000' as your postcode
'003' as your area code

There is also one possibility that when the new headend was created the headend Version number was not advanced. It would explain why it works for Darren on a fresh install, but not on an existing TiVo.

If your in a hurry, A fresh image and GS will work (as Darren has tried). But if you can wait a little while I'll look into it.

Peter.