Thought worth updating this.
I've found a pair of primed shells from Traveinlite - £29 for a pair. Bought through their ebay account - slightly cheaper than on their website.
Removing the old unit.
Remove door card.
Unplug electrics
remove the rubber bung
unbolt the wing mirror
feed the cables through the hole in the door.
Now I've got the mirror in my hand - seperated from the van.
Next stage was interesting - the metal bush is broken - and the cables go through it. The cables have connectors on the end, one of which is too big to pull through. the mirror itself clips in, so it was easy to get that off. To release the broken mounting I had to take the whole lot apart, including the spring. I rolled up the attena into a tube, and fed that through (in the direction from outside to inside) and then fed the heated mirror connect through- from inside to out side. I didn't see way to do this with dismantling the lot! I'm sure I've made a mistake here - but at the time I couldn't see a another way.
The broken mounting is less than a tenner - and on its way to me.
I can foresee I'm going to struggle to compress the spring when it comes to putting it back together. From what I've read it looks like some strong cable ties will do the job - TBC.
Any advice gratefully recieved!