I had the same issue with my girlfiends 03 Micra. The garage kept 'repairing' for about £120 a time, saying they were replacing the switch mech etc. It has recently gone again, so I had a look myself.
I followed what 'needawin' said. GREAT STEPS by the way. However, the boot wouldn't open when I followed this step:
"Now get a small screwdriver and insert it into the female side of the white plug ( the side that is not going to the boot handle)"
After thinking for a while, and looking around, I came to the conclusion that there doesn't seem to be power. So I unclipped all the wiring in the boot section, and found it all led to the top where the boot hinges to the car. I opened the boot manually, by moving the little white looking clip on the lock mech from inside the car. When opened, stand facing the opened boot, and look at the top right (where the boot hinges to the car). There is a rubber wire cover. I pulled this back, and low and behold wires were loose.
I reconnected them and "voila", the switch opened.
It looked like this was a problem previously, and some mechanic had tried to solder back the wires, but they had come loose again.
Sorry for the long winded explanation, but I thought I would put this information here so that if others try 'needawins' steps and it doesn't work, then there is still hope before the garages start to rip you off again!!!