Got my own answer
The door is part of the central locking system.
A plastic rod shoots out from the body and goes through the plastic loop on the door itself.
That plastic rod got stuck (years of ignorance).
Solution?
You can reach it witha long knife from above, and push against the tip of the rod to achieve a release.
I cannot live with this not working next time, so made a breack in he loop. If the rod gets stuck, only a little force will pull teh door open.
David Swarbrick, July 2006