Thanks for the, erm, helpful suggestions
I've eventually managed to get the door open by destroying the lock assembly with the help of a 50mm hole saw, a screwdriver and my trusty hammer, without doing too much damage. There wasn't enough room to get a drill or grinder through the existing holes in the inside of the door so I had to make a new one!
(in case it isn't obvious, I'd already removed the internal panel/trim)
Here's how the procedure went
I took a measurement from the open front passenger door to find how far down the catch is relative to the inside window seal (about 24cm)
I then measured to the same point on the locked drivers door and drilled a pilot hole 30mm from the internal door seal to allow for the 50mm hole saw
Then I got stuck in! When the saw had cut through the metal of the door it had already partially cut into the lock assembly, which is pretty much flush with the inside of the door
I was then able to drill out a bolt which was exposed in the lock assembly and generally bash around with a big screwdriver and hammer. It took about 10 minutes of mashing into the lock and removing the motors, gears and levers until I was able to pull open the latch and release the door
As a bonus, which is good luck rather than good management, there was enough of the latch left to allow me to lock the door from the inside by hand. Now I can still drive without having to tie the door shut while I wait for replacement parts to arrive
However, the central locking system no longer works. And the control system thinks the front drivers door is open (a fair assumption on its part!) so the internal lights stayed on until I pulled the fuse. I'll hopefully get that sorted when I fit the new lock
So all in all, it wasn't too bad to get the door open. Took about 15 minutes and hopefully, the only damage will be the 50mm hole in the inside of the door which will be covered by the trim anyway ( the hole will actually come in quite handy for fitting the new lock!)
Only problem is, I'll never know what caused the problem in the first place as I destroyed all the evidence. Hey ho!
Cheers again for all your suggestions folk