Thanks Andy. When I went on-line to check my booking with BA a couple of weeks ago the API was mentioned then, but they didn't contact me specifically to ask for it, though presumably they would have done when I went on-line to check in this afternoon, if not before.
Made me laugh as with BA you can check in 24 hours before your flight, not a minute earlier - they sent me an e-mail 10 minutes before the time so I had my finger poised.