Many thanks/ You saved the day!
Can I make a couple of comments?
If you are about to do this -
The release tag for the headlight unit (which appears to be labelled in braille) needs to be raised, just a little bit, to allow the hooked metal bit to be pulled towards to opposite light unit.
(Obvious when you have done it, but as it will be stuck fast, you need to know what is supposed to be happening)
When the unit is out (and still attached by the wires), get a mate to well lubricate the slidy bit which is part of the hook. It's quite a large plate which locks the whole unit in place.
The light unit actually locates on the bottom black plastic plate with loads of locating slides, so you might get aforementioned mate to lubricate all of them too. (Remember you are going to do this at least once more - might as well make it easy! ) A pipe cleaner with some lubricant on it would make a good job, I think. Then you can wipe off the spare lube before it attracts all the grit amd grot.
No mates? Best to disconnect the wires - without breaking anything. How, I've no idea.
For what it's worth, to lube plastics and most other things, I use Waxoyl or similar let down with white spirit to make it runnier, and kept in an old spray bottle. It's messy, but leaves a thin film of vaseline-like lube behind when the spirir evaporates.
Hope this helps