I followed the door latch removal steps I found here - whoops, not allowed to post it, search for 'Door lock repair'

Then removed the micro-switch and used the Dremel (1) to cut off the locating/fastening part (2) to graft onto my new micro-switch (3). I used epoxy to secure it and then adjusted the micro-switch 'clip' (4) to ensure it was held tight onto the door latch assembly. Photo (5) is the adapted new micro-switch ready prior to assembly.

Door works great and no error messages - so far.

With my new knowledge of striping the door and latch down I moved onto the rear door as this had decided it would not unlock from the outside - so clearly a problem lurking. I found that the latch circuit board had two dry joints ('cold joints') - hopefully you can see in the photos (10) (11) - zoom if you can and look at the dark ring around the joints - right in the centre, 6 joints - its the left 2 that are faulty. I soldered these up and the repair was complete without issues.

And for some clarity I've added some pictures of the lock itself and the original micro-switch that was faulty.