Thought I'd let you know how I got on... in the end, it was just crud in there that spraying from the ouside (with the handle raised) couldn't reach.
For the benefit of anyone who's never seen the mechanism, what I did was;
1) Remove door card.
2) Undo the two silver bolts at either end of the handle (the rearmost one is reached through a hole just large enough for a socket to fit through)
3) Remove the
outer part of the door handle - basically the triangular trim - allowing access to the top of the mech from the outside.
4) Remove the three bolts that hold the lock mechanism in (on the "end" of the door).
5) Wiggle the mechanism just enough to see the rearward hinge.
6) Mask the electrical gubbins with cloth, then spray the top hinge and spring with degreaser like a maniac. Allow to stand for 5 minutes, moving the handle occasionally.
7) Mop up any excess degreaser, then slather the hinge and spring with lubricant of your choice. Protip: I don't recommend KY jelly as that's water-based.
8) [Haynes]Reassembly is the the reverse of removal. [/Haynes] Ensure that the lock mechanism lines up with the strike plate correctly.
Peasy.
As a side-note, the MOT bloke said that he would not have failed the car for it, as long as he could get the door closed and didn't have to use excessive force on the handle. I asked him what excessive force meant and he replied, "Whatever I say it is!".
Of course a door that won't open from the outside is an immediate fail.