As I have now fixed this I will respond to my own question for someone else's future benefit.
There is no internal release switch. The door lock had become stuck, so basically I had to prise the door open using a screwdriver and then replace the locking mechanism.
1) Remove the rubber hood from around the opening by removing the metal loop around the seal (it is joined with a spring so should come off fairly easily)
2) Undo the screws at the front of the door catch, and reach inside.
3) Unclip the locking mechanism from its mounting bracket (I had the top off the unit so I could see inside as I was doing this, but you cannot easily reach the lock from the top which is why it is necessary to go in from the door opening).
4) Pull the out the cable plug from under the lock and remove the lock.
5) I replaced the locking mechanism, but I found that the old one freed by whacking it on the floor a couple of times. You could try this but I suspect it wouldn't last long before it jammed again.
6) Clip the new unit to the mounting bracket. This cliped back very easily, but again you need to see via the top to know where it is.
7) Screw the new lock in place at the catch area.
8) Plug the cable back in the bottom. My new unit came with an additional microswitch at the top by my model did not use this. You may need to plug another cable in on your model.
9) Refit the metal loop around the seal.
10) Put all covers back on.
Andrew Rubotham, October 2005
