They apparently can only inspect a belt that is fixed with bolts eg! Front seat belts had to be fitted from 1967 and rears from 1987. I reckon you were lucky -it doesn't seem to say this date in the online MoT Tester's manual!
While looking, I happened to see the Securon website
http://www.securon.co.uk and they make belts for Type 3s. 500/30 seems the size and list price is £100 each. They say you should see your supplier (car accessory place) as their price will probably be more favourable. You can choose your colour - almost anything but black !
Years ago I bought up the last stock from a local ASDA shop - some had short stalks for the Type 3 and some longer (they worked in the camper). They were about £5 each!
Incidentally, the same size is used in Beetles.