Do some aerobics with the hazards switch. It rarely comes to work - and then the switch contacts are prone to get corroded.
The hazards switch has a dual function : it connects both sides blinkers circuit together - and connects these to the (standard) blinkers relay.
Seems to be one side switch contacts corroded - or the swich multipin conector not fully homed - on rear of the switch.
I'm not sure your MOT tester will bother check it out - he will be much more interested in the normal blinkers function.