1.8 Focus Estate on an X plate. 98k miles.

If I try to lock it with the magic key, sounds like all doors lock, then a
half second later, they all unlock again. Same if I try to lock it manually
from outside on the driver's door. Same if I try to lock it from inside
using the "LOCK" rocker in the handle. However, if I lock it from inside
using the rocker on the passenger door, it stays locked. Likewise, if I
manually lock it from outside on the passenger door, it locks, and the alarm
sets, as denoted by the indicators flashing twice.

I'm initially thinking that the "I'm locked" microswitch on one of the door
locking mechs is not operating for some mechanical reason, or has suffered a
contact failure, but then, it occurs to me that if this was the case, it
wouldn't lock properly from the passenger door either - unless perhaps the
fault is actually *in* the passenger door ... ?

Any thoughts anyone ? Am I on the right track with my thinking, or
completely off ? If anyone agrees that it's a problem with one of the
internal door mechs, any simple way to determine which one ?

Not desperate to get a quick fix, as it's only me that drives it, and I at
least know that it can be made secure by locking from the passenger door,
but obviously, would like to get it sorted in a reasonable time, so that I
can use the magic key again, which is a much more convenient way of locking
and setting the alarm.

TIA

Arfa