i'd just like to say thanks to Freddyfinn for his little off-the-cuff comment posted above regarding deleting the existing tuner registry keys if installing new tuner drivers.
I had never had this problem before in MCE 2005 and I've been deleting and reloading tuners for 2 years

but upon rebuilding my Vista machine on the weekend I just could not get the tuners working in Vista MC.
Anyway I revisited this thread, read that comment, deleted the two existing keys and Hey Presto!
Off the top of my head the keys are found here:
HKLM\Software\Microsoft\Windows NT\CurrentVersion\Media Center\Service\Video\Tuners
The tuners are listed as sub keys with UID names. Delete them or export if you're unsure, then return to MCE and rescan for channels.
Thanks again
Julan