BFB. Thanks. That explains why the problem started to unfold at 14,000 feet.
In so far as I was able to ascertain it was not so much that the door was not closed, but that the inflatable seal around the rear internal cargo door had failed to inflate.
I'm sure that all the merchants of doom who read this board will be dissapointed to hear that here wasn't a single 'speeding fire engine' in sight. The Captain kept the passengers informed all the way through and sadly again for vultures, there were no 'terrified passengers'.
As I said in my original post, a simple event (hardly and emergency) dealt with very professionally by the crew.