Thank you to everyone who replied. The photos of the door locking mechanism answer a couple of questions that puzzled me as it is not possible to see through a steel plate and the drawings in the parts book are not at all clear. However, it is actually the lock that the key goes in which is the problem. I need a replacement. If you have one, Simon, I would certainly like it. Nick Benwell almost certainly has more than one among his spares, but he is rather elusive at the moment being busy with more urgent and profitable business.
A local locksmith has explained that given a lock and the correct blank, he can make up a key; I now have two new blanks courtesy of eBay. So I am approaching the solution but not there yet. If anyone else has the problem, removing the lock from the door means that you can use a budget lock key inserted into the square hole in the little internal locking piece. One would be desperately unlucky to encounter a car thief with a budget lock key in his pocket.
Stuart and Andy