When I ran a WC-52, I weighed it and it was 2244 kg unladen, therefore 2994 gross. A 51 without a winch must be 150 kg lighter at least so it does need an MOT. You'll get away with it ok untill something goes wrong! The other thing to note is, the exemption applies to 'goods vehicle, pre 1960, over 3500kg gross, used unladen'. If you carry a load, you forfeit that exemption.