Hi Mel,
This might be a bit late but I've just found the same problem on my 2000 megane coupe. With the boot lid open, use a torx screwdriver to remove the motor for the lock mechanism. You should see a small motor driving a black plastic gear. On mine I can see that one of the teeth on the gear has broken, causing the mechanism to seize. If this is the same on yours then the best option is probably to find a nearby breakers with a megane and do a swap.
Hope that helps,
- Sandy
Sandy, December 2008