Yep its an MoT failure.

Sounds like its a bad connection between the Headlamp switch and the bulb. The flasher uses a different switch. But these wires must be joined on their journey to the bulb.

I f you have got a workshop manual with wiring diagram and understand basic electrics you could try and trace this wiring. i just found a similar issue with a motorcycle, turned out be be a connector block behind the headlight.

hope this helps