Right, here's a quick guide on how to replace the door mirror assembly on the Civic:
I've just fitted my wing mirror and all has gone well. It's a bit more complicated than I thought as you DO have to take the interior door panel off to get to the two lower bolts.
The door mirror assembly is held on to the door by three nuts on the inside of the car, which thread onto three bolts the door mirror assembly on the outside.
Start by taking the tweeter panel off from the inside of the door. You do this by pulling it from as close to the top as you can (it's hooked in at the bottom, so don't try pulling the bottom or you'll break the clips). You will then see one of the door mirror nuts quite clearly (it's the smaller of the two). If you peer down the slot underneath the panel you just removed, you'll see the other two nuts - quite difficult to reach but may just about be possible?
To take the door panel off, you remove the two screws behind the plastic cover inside the silver door opening handle area AND the screw which is under the rubber bit where you'd pull to close the door. You then start at the BOTTOM and pull hard to undo the clips until the door panel is hanging just by the very top (no clips). You can then just lift the entire panel up to unhook it from the door (it's hooked into a slot on the top edge which the glass goes down). Stand the (now loose) plastic door insert on a box so it's not being supported by it's own cables. You don't have to solder or disconnect any of the wires (other than the one that goes to the wing mirror).
It took me about 20 minutes to do the above - including putting the new one back on and putting the door back together again and also swapping the covers between the replacement mirror and the old one (see below). It's saved me £££ in Honda labour so I think it's worth doing and is fairly low risk. The only risks are, that you might break a couple of the door clips when you pull it out, but these are fairly cheap if you DON'T buy them from Honda and I've not broken any myself.
Further notes on mirror disassembly:
To remove the mirror glass, just push the mirror on the outside edge (furthest from the car) so it swivels round giving your fingers access to the inside edge, then just pull the mirror out carefully with your fingers from the inside edge (do not use a screwdriver or any metal tools or you'll shatter the glass!). You can then disconnect the wires to the mirror heating element.
To remove the body-coloured shell, remove the mirror glass as above, then use needle-nose pliers and a screwdriver to release the clips which you can see from the reverse side. It's a bit fiddly, but eventually you should get all the clips undone simultaneously and the cover should come off when you tilt it forwards.
If you need to take the mirror apart any further, there are star shaped screws which you might need to undo. Personally I found I could undo these with a flat bladed driver quite easily - it's just the matter of finding one which exactly fits (if it's slipping and damaging the screw then stop before you ruin it!). Do not ever attempt to use a cross-head or Philips screw driver in a torx or 'star' screw or you'll ruin it and you'll be stuck!
Hope this helps someone!