I've now solved the problem. In large part my fault but also perhaps a poor computer interface.
I normally book flights with one credit card and when asked by the BA system for the number of the card used inserted this. When I was eventually able to track down the e-ticket receipt on another computer I realised that I had used a different crd for this booking. That was my fault, but I wonder why the BA system could not simply have said - wrong card number - rather than presenting a message which suggested -at least to me - that the system was down?
Thanks for the responses to my query