Curious.
You said they pulled you over and had a good look round your car.........before mentioning the seat belt issue? There was no other suggestion of infringement(s) being the reason for stopping you before the seat belt issue was raised?
Not, first stating "we have stopped you because you were not wearing a seat belt" and then had a look round the car?
And did you stay in the car until the officer approached, or get out before he got to you?
And did they at any point see you from the front or the side before pulling you over?
And it was nightime?
Have you been done for that before or any other car/driving infringement, or an otherwise clean licence? (Maybe you may not wish to reveal that here)
Any chance you might have passed a cctv that might verify your wearing the belt, just before the pull?
Because as others have said, It would extremely difficult to see through the rear of the car that you were not wearing a seatbelt and putting it on, (or taking it off) immediately upon being stopped.
Think this would be the scenario I would raise with a good Brief if you were so minded, as it rather sounds as though they were looking for a 'tug' and that was their fallback option.