Thanks everyone. Yeah you are right it looks like they are very difficult to get apart.
What we managed to find was an electric folding one from a Freda which luckily has screws on the bottom (I presume this is a gen 1 design) which you can use to take the bracket off the bottom, this then means the guts slide out of the mirror casing.
We then cut the bracket off the existing mirror casing (which was still OK) and pulled the old guts out, transplanted the new one and put the screws back at the bottom.
The wiring plug looks slightly different which worried us at first but plugging it all in it works fine - it's a different design electrically inside the mirror but still works perfectly.
So crisis averted!
And yes Simon mine is exactly the same colour!

I am already going to start stashing a few spares just in case...