Well after a large dose of courage I have managed to replace the standard mirror with the auto dimming version. Only failure was the photos (sorry), so hopefully the instructions below help. Completed in 20 minutes.
Please PM me if you need more info.
The steps I used are as follows, and use the picture shown at the end of this post.
Make sure the car is warm inside so the plastics are not brittle. This is very important as my first attempt when the car was cold I broke one of the the tags off the plastic cover.
Remove the plastic cover that sits between the head lining and the mirror (shown as item 5). I used two old credit cards either side to raise the cover slightly (the place I used is on the diagram. It's where the arrow from the circle points), and using a thin plastic blunt blade inserted between the head lining and the cover, gently lift of the cover. All going well it should come off without breaking any lugs. As my replacement mirror had a new cover, this was not a problem for me, but take your time.
You will see a cable connector clipped in on the right hand side of the mirror main housing located on the windscreen, gently pull this out noting how it clips back in. Once out of its clip, disconnect the cable, but first make a note of how it connects either using a marker pen or masking tape. The connector once disconnected will leave the biggest part of the connector block on the mirror.
The next bit is easy but just be careful. Holding the main body of the mirror, twist the assembly anti clock-wise, slowly about a quarter to half a turn, then the mirror will come away from the clamps on the windscreen. You can breath again....
To replace the new mirror.
Reconnect the cable to the connector and refit the connector block back in to the clips on the main housing. Make sure the connector sit correctly but don't worry if it moves about as you can re-fit the connector block back in place before refitting the plastic cover.
Next offer up the mirror so the locating lugs are engaged (the new mirror needs to be offered up to the clamp between quarter and half turn left of centre.
One the lugs are engaged, push gently and rotate clockwise until the mirror is fully straight and is firmly attached. Another deep breath after this, and almost done.
Once you are happy, check the connector block is still ok, and if not, refit this.
Before fitting the plastic cover switch on the ignition, and all going well the green light on the mirror will glow. If not, press the button as it may be in the off position. If light goes green, then success.
Refit the plastic cover carefully, and job down.
I have light and rain sensors, and electronic climate control on my car. The part number is 8X0857511E. The mirror is available in grey or black, so you need to ensure the dealer orders the correct colour. Without light and rain sensors, you will need to check the wiring first and that the part required has a different part number. Best to ask the dealer before ordering.