Ok, I've now managed to cut the o-ring loose. Does anyone know the specs of the o-ring? The o-ring pack I bought at Repco doesn't seem to have an exact match. The original seems to have an ID of 22.5mm. The cross section is about 4mm wide and about 3mm deep. That being said, I strongly suspect that it was originally circular in cross section, it has just lost most of its elasticity after 15 years.
I guess at least you can get the o-rings from the dealer fairly cheap in the UK. (Though they erroneously suggest that you would need to remove the water pump, which you don't.)
http://www.micra.org.uk/threads/15422-Mum-s-Micra-is-losing-water...FAST!
To further flesh out asdame's instructions, here is what I did (or will do):
Step 0: Order a N*210492J200 O-ring from your local Nissan dealer, they may have to order it in, so do this in advance. It only costs about $3, so go genuine. (Thanks kovert.k11!). Also ensure that you have enough coolant (and distilled water, if you use concentrate) for a coolant change.
1. Jack the front of the car up so that it sits on jack stands at the front jacking points (e.g. the sills).
2. Remove the 3*12mm bolts on the Y-shaped inlet manifold bottom support bracket for easier access, and remove the bracket. You will need to be under the car to do this.
3. Remove the 12mm bolt holding the return line to the back of the engine block.
4. While we are under the car, at this point it is probably a good idea to remove the hose at the bottom of the radiator, to empty the coolant (e.g. into a bucket such as used for changing the oil.). You will probably want a Philips head screwdriver to undo the worm drive clip.
5. Now, we will want to remove the 12mm bolt near the thermostat, which is tricky because there is some stuff in the way. We will want to first remove the bolt (10mm?) that secures the black air intake pipe not far from the battery.
6. Then we will want to remove the coolant pipe that goes from the thermostat to the firewall, otherwise it will be very difficult to get a ratchet in there. Might want to give the inside a clean as well. You will need a regular screwdriver to unscrew the worm drive clips.
7. Unscrew the 12mm bolt that holds the coolant return pipe to the thermostat housing.
8. Now we are free to pull the coolant return pipe from the thermostat housing end, so that it pulls straight out of the water pump housing.
9. Cut the o-ring out. I used box cutters.
10. Use a small wire brush to clean the corrosion/dried coolant off the end of the coolant return pipe where you have taken it out. Probably also would pay to brush the inside of the water pump housing (inserting the brush, and then pulling the corrosion out with the brush so that you aren't spreading the corrosion inside the coolant loop.)
11. Replace the o-ring, grease with something e.g. engine oil.
12. Reassemble. Refer to Haynes manual for coolant change procedure. The original manual might have it too, not sure.
