After much faffing around & borrowing the wife's Sten when underground, this was finally taken apart yesterday.
The unit itself was fine, no problems with corrosion or anything else. One strand of cable had broken at the point where it enters the unit, that is, where it gets worked hardest when the unit is tilted. Cable shortened, re-soldered & the light now works perfectly again. I reckon it has stood up rather well to a fair few years of use and abuse now.