Hi guys, my Gp Mk2 Buggy does not have rear seatbelts and I wish to fit seatbelts. Any suggestions please ? How and what?

You need a metal frame behind the shell or an extension of the roll bar around the inside of the tub. Use the site search engine the subject comes up every now and then.

Does your buggy have a roll hoop?   I have seen a Doon with 2 metal "flanges approx` 4mm thick welded to the tube upright close to the floor. Same for both sides. If long wheel base you will already have the original tunnel threaded attachments. If short wheel base you may or may not. If not plates with threaded nut on each available to weld to tunnel.  The hoop will need a threaded stub welded on inner facing surface of the roll hoop at suitable height each side. (This mimics a "B" pillar location). This allows a 4 point retractable standard seat belt to be fitted. (Retractor at floor end). A bit of work but do-able.

As Keith says, (snoopy).... If you use some tubing under the "rear seat base area" you can use a harness. (much simpler option).
Tube can fit to the rear casting  at the rear in existing body mount locations etc. The rear belt of harness can then be bolted to the tub at back through the square tube. It`s really important that the rear harness section is not bolted to the floorpan behind the seat as this creates too severe a angle and serious injury can occur in an accident.

Put pictures to these words by visiting the Securon website. Shows all belt options and gives diagrams for each style of fitment.
Only use standard specified nuts\bolts\spacers etc that are specific for this use.

Does your buggy have a roll hoop?   I have seen a Doon with 2 metal "flanges approx` 4mm thick welded to the tube upright close to the floor. Same for both sides. If long wheel base you will already have the original tunnel threaded attachments. If short wheel base you may or may not. If not plates with threaded nut on each available to weld to tunnel.  The hoop will need a threaded stub welded on inner facing surface of the roll hoop at suitable height each side. (This mimics a "B" pillar location). This allows a 4 point retractable standard seat belt to be fitted. (Retractor at floor end). A bit of work but do-able.

As Keith says, (snoopy).... If you use some tubing under the "rear seat base area" you can use a harness. (much simpler option).
Tube can fit to the rear casting  at the rear in existing body mount locations etc. The rear belt of harness can then be bolted to the tub at back through the square tube. It`s really important that the rear harness section is not bolted to the floorpan behind the seat as this creates too severe a angle and serious injury can occur in an accident.

Put pictures to these words by visiting the Securon website. Shows all belt options and gives diagrams for each style of fitment.
Only use standard specified nuts\bolts\spacers etc that are specific for this use.

The roll hoop and standard fixings in the tunnel are too far forward to be of use for mounting rear belts.

David is right here. Apologies for not reading the query properly. I missed the "rear" part of it completely.