Ay up,
I fixed this problem about a year ago.
The mechanism consists of a electric switch, motor and catch. When you press the handle on the boot it closes the switch, turning on the motor and moving the catch. Unfortunately the switch is prone to getting dirt inside and stopping it from working. The switch costs about £20 and it quite easily changed. You have to remove the plastic on the boot lid to get to the lock and switch. Its all explained i the haynes manual (has been my life saver a few times). I suggest if you replace the switch, insert a piece of plastic shopping bag between the switch and the handle. This should stop any dirt getting in. Not had a problem since.
G