Mirror discolour
 

A few days back I noticed a blue colour spreading across the nearside external mirror from the outer edge to approximately the middle of the glass. After a short while this cleared and has not since reappeared.
The mirror is an anti dazzle one.
Any ideas, please?

Starting to fail I'd wager.

Yep, I'd agree that doesnt sound good. I had one that looked like oil was being poured in between the surface and the backing from below and it gradually spread. sounds like you may need to go on the "a8 parts waiting list for auto dim mirrors" lol

Get the part number and start browsing eBay. ;)
They are very expensive (even used) but the relatively good news is that it's the drivers side that usually fails (collisions?) so there are more passenger side ones around.
(I eventually got my drivers side one by buying a LHD passenger side one from the USA.)

Just FYI, the chemical liquid they use to make the dimming work is very very corrosive and if it leaks from the mirror it will eat any paint or plastic.

When mine went, I must have been in the right place at the right time. I managed to snap up a pair of the newer style which do not use the chemical for not much more than £70 for both. They are genuine Audi, but do not have the thick black border of the early ones and I think are from the pre facelift, just before the first facelift unless the first facelift used the same mirror casing as the latter ones are a different shape.