I bought from the site Marlon recommended, fitted it - works OK. Not bad for a quick fix @ £10
In the interests of full disclore there are a few negatives:
1. It doesnt fit perfectly, there's a clear difference between the cut of the glass on the OEM vs the replacement. I think the OEM 'slotted' in better.
2. The mirror glass is a little strange - i actually find it harder to focus through the replacement than OEM, almost like a cheap mirror or something. It might be that it has a strange angle on the bend (for blind spot) that gives it a weird out of focus feeling.
I'm not sure if my previous glass was 'connected' to a heated mirror wire, i can see a wire under the old fitting (it's also hard to tell what the old fitting actually is, vs which bit is glue resedue, or which bit is a film that was once stuck to old glass. Really not that clear.
FWIW i'll most likely ring and order from BMW and see if i can get a better finish.
JB