Mirror now replaced, very easy job.
As far as I can tell in the daylight, the new mirror appears to dim significantly more then the centre section of the old mirror. I can only assume that it's been on its way out for a while.
There was no shadow, rings, bubbles or blurry sections on my old mirror, it just does not dim correctly.
Here's a picture of the reflection the nice anaglypta paper on my study ceiling! The light variation is because the light is in the middle of the room, to the left in the photo.
And a 'photoshopped' version of what it looked like when dimmed.