[HPsauce]

Originally Posted by HPsauce

I've found a possible reason for this, though I haven't yet had the latch off the car to verify.

Possible is now definite.
Just swapped the latch over, actually leaving the existing microswitch in place - it was easier that way and it looked and worked perfectly fine.

The old latch has the same plastic piece inside, though it was made of dark purple-blue plastic not cloudy white nylon. And it was broken in exactly the same way. So, clearly a design weakness.

Looking at the way the latch is mounted it has space around it for water to drain out and the microswitch is "high and dry". So I think I just had the broken plastic problem, the water ingress was a red herring.

So, something for people to look out for. If your boot latch starts misbehaving it could just be a tiny piece of broken plastic inside.
You can, if you look very carefully, actually see the plastic piece by looking at the latch mounted on the car with a torch. If it's not visible it's broken off and you have, or will probably have, a problem. I'll try to post a photo if I can, but that may be difficult, so I'll describe it:

Open the boot, and with a bright light look closely at the latch in the lid, from end-on.
You should just be able to peer down the crack between the plastic latch cover and the boot lid metalwork on what (with the boot open) will be the underside (i.e. inside).
Slightly offset to the offside (towards the lock) you will see a yellowish metal arm (approx. 5mm wide) protruding from a slot in the latch body.
This arm has the lock release cable attached to it, though that won't be visible. If you operate the unlock button (e.g. on your remote) it will move slightly.
Through the open (lock) side of that slot you should see a small piece of plastic extending right across, at a bit of an angle. It also flexes very slightly when the arm moves.
If it's not there or appears loose that's your problem. But it is a very narrow gap so you might not see anything and need to dismantle it to check.