If your boot wont lock at all I imadgine you have the very same problem I had untill this weekend.
You probally already know the locking positions of the boot key slot, but just encase horizontal with the key turned fully to the right = always open, vertical = alwasy locked and horizontal key turned all the way left = unlocked/locked depending on central locking.
If you pop off the plastic panel on the inside of the boot lid (held on my 5 push clips) you will reveal the back of the lock and the wiper motor etc. on the right hand side of the boot panel there will be a blue 'pod' with a rubber hose coming out of it followed by a brass looking rod which doubles back on itself and runs from right - left to the center of the boot and onto the mechanism which connects the push button on the boot to the lock/catch.
The rod runs through this mechanism (behind the white hooky thing) and out the other side. It is responsible for pushing the white plastic clip from side to side to lock/unlock the boot (technically its always unlocked but when the white hook is moved the button does not come in contact with it so you cant open it)
I had to use about a pint of WD40 and wiggle the long bar from left to right for about 5 minutes to free it up as mine was stuck solid. It would try to move and buzz etc, realise it was seized and give up. Which sounds like what is happening to you?
I found it helped to put a screw driver between the left hand side of the piece of metal which the white hooky thing sits in because without it the whole mechanism started to give a little instead of just the bar which goes through it.
After finnaly getting it loose and moving it back and forth whilst spraying in WD40 it now locks like a charm.
Hope that helps with the boot issue at least?
