It's the first used date that is important, not the build date.
From 1/1/65 onwards it was front seat belts only, until 1/4/82, when rear belt anchorage points became a requirement. Actual rear belts were not compulsory until 1/4/87.
Prior to 1/4/81, the front belts could be two point (diagonal only) fittings, with no lap belt.
Where two seats are fitted in the rear (a seat is generally 400mm of seat space i.e the EEC "bum" width) a three point
inertia reel belt is needed in one of the seats, doesn't matter which. If it hasn't got that then each seat must be fitted with either a lap belt, child restraint belt, disabled persons belt or three point static belt.
Where three seats are fitted in the rear a three point inertia reel belt is needed in one of the outboard seats and one of the remaining seats with a lap belt, child restraint belt, disabled persons belt or a three point static or inertia reel belt.
Alternatively a three point static belt can be fitted to any of the seats and one of the remaining fitted with a child restraint or disabled persons belt.
If none of the above are met then every seat in the rear must be fitted with either a lap belt, child restraint belt, disabled persons belt or a three point static or inertia reel belt. These belts can be fitted in any combination.
As a general rule therefore, from 1st April 1987, rear seats need one belt if two seats are fitted and two belts if three seats are fitted. Remember these are the minimum legal belts that must be fitted but all belts fitted at the time of test must comply with the requirements.
Vehicles first used before 1st April 1987 do not need seat belts in the rear but if they're fitted they get tested. Vehicles first used before 1st January 1965 don't need any seat belts but if fitted they get tested.
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