Oh, deep joy. In attempting to make a booking on ba.com earlier (flight + car, so now BAH), the website crashed at the confirmation stage and kicked me back to the home page. I didn't receive a confirmation email, so replicated the booking process, which was successful at the second attempt. Then I received a further email, showing that my first attempt was indeed successful.

So I have duplicate bookings, neither of which can be cancelled online and BAH are all-but impossible to contact by phone.