In case it proves of use to someone...
If the blue main beam indicator / tell-tale light on your dash isn't working, which may result in the above MOT failure as I found out, then this is the bulb to check.
There are reports on other threads that it is a surface mount LED - that's true for all the other indicators in that cluster, but the main beam indicator is a changeable bulb (FL at least, can't say for PFL).
Check the simple things first (note to self - must check the simple things first!)
(https://i.ibb.co/87Z6yR9/dash2.jpg) (https://ibb.co/bK9z4yX)
Talking of tell-tale, I was surprised to note there is no indicator on the dash for side or main lights.