An update on progress (or lack of it!) today.
I decided to destroy the card, rather than try to pull back the original, so I checked the availability of a replacement, ordered it and then set to with a Power file. I cut a hole in the card a bit larger than the hole in the door frame.
The first thing I found inside, is that the external handle is not connected to the lock. The rod from the handle is just hanging in mid air.
Having done all this though, I have still not yet managed to unlock the door. It seems well and truly jammed in the locked position. Even though I can now see the full length of the locking tab rod, It is impossible to pull it upwards to unlock the door. I guess this is why the central locking cannot open it.
Having read again the posting on a similar topic from the link provided by Andy B (above), it appears that there is a nut on the side of the lock, so I am hoping to be able to turn that tomorrow (providing I can find it!), unless there are any other suggestions on how to free what appears to be a very solid lock. I have owned this car almost since it was new, so I am confident that it has not been tampered with previously and all the mechanism is the original.
All this problem started when the door would not close properly. It just kept bouncing back. In the end, I locked the car with this door open and then shut it.
To date, it has not opened since.
If there are any other suggestions they would be welcome!