[Clifford Pope]

Which bit won't work? Do you mean when you turn the key, the master pop-up lever doesn't go down, that it goes down but the slave ones don't, or nothing happens but you can hear it trying?
What happens if you manually depress the main lever - do the other doors lock then? If so, then it looks like the key-operated switch.

Points to check:
Key-operated switch
master switch on main control button
all the slave servos
all the linkages
don't forget tailgate, which is very prone to sticking, and to damage or displacement of mechanism by people slamming it.
fuse-holder for corrosion and poor contacts.
Poor action anywhere in the 5 servo points tends to affect the rest of the system.

If there is no clearly obvious fault then you just have to painstakingly work round the whole system, testing for voltage and freedom of movement.

Hope this helps,

Cliff pope