Good question Micha.
For our purposes the basics are:
Vehicle first used before 01/01/1965 - no belts required.
Vehicle first used between 01/01/1965 and 31/03/1981 - two point diagonal belts required for front seats.
Vehicles first used after 31/03/1981 - three point, lap and diagonal belts required for front seats.
http://assets.dft.gov.uk/publication...e-vehicles.pdf
The dates when you actually had to start wearing them are a little different, but, having limped away from a head-on crash on 1st April 1983, just four months after our law changed, I wouldn't want to be without one.