With a little advice from @
edgeoftime I've got into it
I'd hoped the missing bit of lens was inside the somewhere, but it's not. Making a repair might be tricky, the damage is more extensive than I feared, but I'm loathed to throw £60 quid at it for a new one.
Complete mirror ***emblies are fairly scarce, and so relatively expensive, so I doubt I'd save much by getting a scrapper and selling on the bits I don't need.
Anyhow, should anyone else need to do it:
Move the mirror glass (either by motor or by hand) so the outside bottom corner is as far forward as possible.
Shine a torch between the glass and the shell and you'll just about see the white support frame.
Use a flat blade screw driver between the frame and the glass (the glass has a black plastic backing plate so it won't damage too easily) to gently twist / prise the retaining lug free.
There's one on the inside bottom corner too, but that released with a bit of twisting (no screwdriver required)
There is a small amount of chewing gum like black goop between the glass and the support frame.
To stop rattles I presume.
This pulled away without too much effort.
The top of the glass is held on with hooks rather than clips, so once the bottom is free the glass needs to be pulled down and out.
I left the de-mister wires connected for what I was doing, but they could be removed if required. A bit of tape might be an idea to stop them shorting together.
Access to the LED indicator module is by removing 4 self tapping screws and drawing the inner shell forward over the motor ***embly.
The indicator more or less falls right out from there. It's somewhat bigger than it looks from the outside.
Re-***embly is just the reverse.
Once the top of the glass is correctly hooked in place slip your forefinger behind the glass onto the support frame and with a pinching motion use your thumb to press the glass into place until the clip engages.