Door lock
 

I finally fitted the locking handle on the passenger side of my 403 only to find that as I turn it the door unlocks itself and opens before the handle locks off the outside button.

Any ideas as to why this should happen?

You may be trying to do it the wrong way round.
The way the locking lever should work is to release tension on the wire that operates the door lock, so that pressing the door buttons doesn't pull on the wire, because it's now slack, and operate the lock.
It sounds a bit as though you've got it set up nice and tight in the 'lock' position (which you may be taking for 'unlock'), and when you turn it to what you intend to be 'unlock', it's increasing the tension further and pulling on the door lock to release it.

You were nearly right Hugh, it was set up nice and tight in the open position. When the lever was moved to lock the cable tightened up slightly and sprang the door before releasing the tension!