So, I managed to get the door open, but I’m not entirely sure what was wrong with it. Bear in mind that before i started dismantling stuff, I tried the door again in case it was just a one off thing. I repeatedly locked and unlocked the door whilst trying, and the button was going up and down on the door. Pulling the handle on the inside of the door also raised the button if locked, but it still wouldn’t open.
So, I disconnected the battery, removed the passenger seat, (heavier than I thought!), then proceeded to remove the door card which was a bit of a faff because I had to slide it out from between the door and the end of the dashboard which resulted in several destroyed plastic clips, however, I had spares.
I noticed that someone had been in the door previously to me owning the car as parts of the membrane were already ripped, so it made me wonder if it’s happened before. Anyway, I removed the airbag and had a good look at the mechanism but couldn’t see anything out of the ordinary. I noticed the button on the door lock was down, most likely knocked by me removing the door card. Pulling it up by hand only resulted in it dropping back down so the door was probably locked at this point. I reconnected the battery and tried the locks via the key fob and all the locks worked apart from the passenger door, so my initial thought was that the actuator was broken, but then remembered it worked fine before I removed the door card. It didn’t take me long to realise the window control box needed to be connected for it to work, so I reconnected it, pressed the unlock button on the key fob and hey presto, I could open the door!
I was baffled and couldn’t work out what had gone wrong with it, but at that point i didn't care, it was working again so I gave the mechanism a good greasing, tried the door a few times then put it all back together. So far, all good.
Only thing now is that since I reconnected the battery to test the locks whilst the airbag was disconnected, the SRS warning lamp is now lit. Yeah, I know you suggested just moving the airbag out of the way with a coat hanger, but I wanted it completely out of my way (and the seat!). I did disconnect the battery again before I reinstalled the airbag and seat, but I didn't know any better as I'd never removed an airbag before. Fortunately, I have access to a Launch diagnostic tool so I’ll be able to reset it.