Pendant backbox - what's going on?

Looks like a very dirty Pyro gland

Ahhh ffs, I think you are right. Well, this job is about to get interesting. The reason the light came down in the first place is because the LED was flickering whilst the switch was off, suggesting some leakage going on. Now I've got this in the mix.

That is almost certainly an MI cable pot. I'm not clear as to what you are trying to explain.

Borrowed N in Pyro to an outside light ?

That MI seal is poking into the BESA box instead of being seated inside the gland screwed into its conduit entry. Advantage; you can see and get to the seal, offset against it hogging the space inside the box. The MI sheath is the earth but shouldn't be serving as the neutral; that practice was used historically (earthed concentric wiring) but not in domestic work. If someone has picked it up as a neutral he's a very naughty boy.

There should be two or three sleeved conductors exiting the black disc in the end of the pot. Some later seals also have an earth tail spot welded onto the pot itself which is securely in contact with the MI sheath, but more commonly the earth connects only via the gland (which you can't see) and box as a conduit would. Try to avoid flexing the conductors too close to where they exit the pot because if they break off, or the disc comes loose, you have a more complex job on your hands.

A failed or damaged seal allows the MI insulation to absorb moisture from the air and its insulation falls. If one end or the other has got very bad or there's a puncture in the cable sheath somewhere, it might have fallen low enough on a switch cable to cause flashes when off. Although, if the MI doesn't contain both the PL and SL conductors, that's not the problem here. If it's suspect, disconnect all cores and megger to the sheath.