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This How Too covers a number of cars using the same glass provided by MirrorJohn. These pictures are from my C5 RS6 which dose not have the auto lights and rain sensor but these are separate sensors to the glass and shouldn’t be involved in the procedure.

Firstly remove your rear view mirror (RVM) from the windscreen. To do this rotate the RVM arm anti-clockwise by 90deg, mine was very stiff but it did pop off. Disconnect the cable that was hidden behind the trim cover. You can see the mounting plate and electrical connector here.
Now you need to split the RVM housing. Start by splitting the glass face surround. Using the appropriate tools pop the surround off. (You can buy an excellent trim removal kit on eBay for very little money)
Once the glass retaining trim is off the glass will move forward, you can then access the connection block. Give this block a little wiggle and it will pop out.

Now you have the mirror away from the housing you’ll need to separate the control board from the duff glass.

Simply use your soldering iron to heat the existing solder a separate the cable from the glass contact point.
Now gently work the board away from the old glass. You can use a knife or fishing line. Whatever you use TAKE YOUR TIME!
Now John suggested checking the light sensor solder at the board, apparently this can be problematic and is worth checking now. Give the sensor a very light wiggle. There should be no movement in the solder or sensor. If there is you’ll need to clean up this solder and reapply.
Now apply double sided tape to the new glass in the same area the board was sitting on the old glass (obviously on the correct side). Before sticking the board down make sure the light sensor is going in its hole.
Now resolder the boards wire to the new glass connection points.
Reconnect the board to the housing, making sure that it goes the right way round.
Reassemble the mirror housing, connect the mirror cables at the windscreen and refit the mirror (this was even harder to refit than remove.

Hope this helps, please feel free to ask any question.

Great How To Chris, I know what I'll be doing this weekend

This helped loads - another happy customer of https://www.mirrorjohn.com - I bought blind after I got no response from the contact page.
A few notes from by experience:

1) there is a piece of trim that covers wires from the headline down to the mirror, remove this first
2) I found it easier to disconnect the wire from the mirror before trying to remove the mirror from the windscreen, you'll need a pen or fine pointed tool to push into the circular hole on the side of the black wire connector.
3) I ended up using a large pair of vice grips to rotate the mirror bracket off the windscreen, I'd recommend lining the grips with a cloth if you want to avoid marking the black metal (I did not avoid marking the metal )
4) when re-soldering the wires back onto the mirror, remember to solder to the top face, not the outside edge, the glass & the metal contact strip is very neat in the plastic housing.