Further to my post above, we took the plunge and bought a new Maxi Cosi Cabriofix seat with EasyFix base. Got it all in for £155 off the Boots website which was pretty good going (some discounts included).
Bit of a frustrating purchase as it is only going to be used for a few more months, but on the flip side at least it gives us some peace of mind now.
Anyway, I did some measurements and basically if your Group 0 car seat is 65cm or longer in terms of depth, it's highly unlikely that you will be able to use it unless you have an Isofix/seatbelt base. The seatbelt just won't loop round the back of the seat.
Still can't believe the Punto has shorter seatbelts in the rear than a 500 or Panda:shrug: