There is actually no such thing as a Cat 1 immobiliser per se.
Cat 1 is an immobiliser AND alarm combination.
Cat 2 is immobiliser only.
So simplistically, if you have no alarm you cannot be Cat 1, but it does not necessarily follow that having an alarm automatically gives Cat 1 status (criteria must be met).
Thanks ABerrant. That is really useful info, and a great link too.
It was written on my insurance renewal letter that the vehicle had to be fitted with Thatcham 1 or 2 immobiliser. I have been insured with them for a couple of years so it made me wonder if they maybe had knowledge that my car wasn't fitted with one?