Thanks chaps. Maybe a wiring problem after all?
After testing in the house, I thought at least the motor and gearbox is OK.
Maybe the hall sensors are duff but cannot test.
I tested the two outer thicker cables resistance again, it was ~2OHM - I think I made a decimal place mistake last time when the multimeter was on the beep function.
I have cut the two damaged thick cables and resoldered them using my last bit of environmentally-unfriendly lead solder. Resealed the connector. for now but want to get one in stock and replace if it goes again.
I removed the big o-ring from the caliper and cleaned it all up and put silicone grease on. Blob of Tamiya grease in the torx-style caliper drive shaft. All seems ok now.
Screws to remove the motor were really stiff and I had to use the mole-grips on a T30 torx. I used blue loctite on refitting screws as it keeps moisture out and de-oxidises (reducing agent). I wouldn't fancy using the torx tool that comes in the car's tool kit! Once the motor is removed, you can wind the park brake caliper plunger back with a T40 torx, that turns VERY easily.
I looked at greasing the EPB gearbox but is is in the motor half that you can't get into so just used a little silicone spray in the exposed side to keep the moisture at bay - I also think grease may damage the nylon toothed belt that is in there.
https://www.youtube.com/watch?v=tAGR1bd_8KQ
https://www.youtube.com/watch?v=ThVKwHJWTLA
For future reference, the loom-side connector part number is 1J0919231
http://www.aliexpress.com/item/Allro...582661446.html
although on this one the outer wires on the EPB are more like 4mm thick so will try and find the right type and replace them.