I had the same issue and it drove me nuts...intermittent LH dip beam out that finally quit. Turned out to be a cleanly severed wire 4" into the wrapped harness over the wheel well. No damage to the harness and not at a flex point....baffling, but someone here nailed it "..it is British, anything is possible..."
Assuming it is not the obvious/simple (fuse, fuse box terminals, headlight plug), then work your way up the harnesses wiggling it as you go.
Oh, and in the middle of this I came across some fuses I had that were bad w/ high resistance out of the package. Look for the simple stuff, but also the weird.
'85 90 2.5NA
'72 SIII 88"
Couple other cars