So, today my new alloy casting arrived. I followed this guy's YouTube guide, which told me everything I needed to do for the job (with the exception of removing the handle actuator, which was easily sorted).
So, here's the new casting in place. I have temporarlily remounted the LED lamps from the old casting, but they will be replaced by the new ones when they arrive.
I used some red rubber grease to help seal one lamp in - but I'll clean it out and perhaps use some silicone sealer when I fit the new ones, as I'm not a fan of the 'crime scene' effect it gave the car.
To answer my own silly question - the bolts are M3 16, which I got from my 'come in handy' drawer. As I'm using stainless screws into an alloy casting, I gave the threads a generous gob of copper grease.
The bolt is really awkward to do up - but I managed to find a use for a little ratchet driver I've had gathering dust for a couple of years, which did the job perfectly with just a hex bit fitted in it.
The only pain in the parts aspect of the job is that the number plate needs to be removed to get the casting out of the car. No problem if it's held in by bolts, but mine was held in by sticky pads. Luckily I had some spares - but cleaning the old foam off was a bit of a pain...
If it wasn't for that, this is a job you could comfortably do in an hour or so. Once you have the old casting out, you can take it away from the car and switch the parts from old to new in the comfort of your house, rather than stretching over the boot lid outside.