Finally managed to find time (and nice enough weather) too look into the locking problem, so thought i'd update...main headline is I seem to have fixed it.
Stripped down the passenger door and pulled out the lock mechanism, then separated the electric part from the mechanical. Next opened up the electrics to have a look for anything untoward.
Had previously done this on the drivers side, and found a dry joint on the circuit board. Soldering this up fixed the problems I was having with that door.
On the passenger side all the joints and micro switches were fine, so pulled the connections from the motors to test them. That's when i noticed one of the motor connections looked a bit suspect.
There are two pins that slide into the ends of each motor to connect them to the main circuit board. Inside the connections on the motors are little copper tabs, which should push against the sides of the pins (just like in a wall plug). On one of these, rather than the pin sliding into the connector, it had crushed the copper tab down.
Took plastic cover off the back of the motor so i could straighten out this copper tab, then put it all back together. There were also signs of oxidation on the copper where the bad connection had been.
I can't say for certain that the poor motor connection was the problem, but it all works again now
Can only assume that either someone had taken it apart before, or it had been like that from the factory. Either way it would have been that way since before I bought the car (over 9 years), so it's taken a while for connection to degrade.
For info the mechanical part of the lock was all fine; nothing sticking or corroded.
Now I've posted this, i bet it'll start playing up again