The left door indicator has stopped working and I can't see an obvious way to get to the bulb. Can somebody help?

You cannot get to the bulb, it's a sealed unit. You need to remove the mirror case and unplug the electrical connector, the indicator section is held on by two torx screws. A replacement indicator unit is about £20, I have just done the same job after somebody clipped the door mirror and smashed the indicator lens. Hope this helps.

I have no entry or indicator lights on this mirror and no visible damage, I have stuffed a bag over it till I can test for power in the daylight!(cover removed)

This thread might be of use to you:

http://www.briskoda.net/forums/topic/232763-wing-mirror-indicator-leds/