Thanks for the reply. I've just sent the Viva off to the painter having stripped it down to a rolling shell, engine out, interior out, dash stripped etc but was defeated by the interior light /mirror!
I have a vague recollection that the mirror stem snaps out or maybe rotates but it it doesn't respond to moderate force.
Sorry mike - at the time I couldnt work out how you could not see the 3rd screw. It seems that some mirror stems cover the 3rd screw - and yes you have to prise that stem out to get to it get a screwdriver blade under the stem face to the light surround and twist