I've been having some issues with our Mk3 mondeo estate, regarding locking.
We were unable to get into the boot via button or the micro swich handle (key was also not working).
The boot handle microswitches fail (as was our problem), and this prevents the button on the dash from working.
So take the panel off the inside of the boot.
Unplug (small black) the handle from the boot wiring.
Now see if the button on the dash works fine opening the boot.
If the dash button works, then you need a new boot handle microswitch.
I bought one from Ford, about £34 (with 10% discount).

Hope this helps.