Thanks to all who offered advice. It worked: in fact, I was at D16 waiting to board BA AMS/LHR around 50 minutes in advance of that flight, after a timely QR arrival in AMS, no delay at security and a sprint by me from G to D (HBO and J throughout). So it is indeed possible to do XIY/PKX/DOH/AMS/LHR/BHD, on three separate tickets in one go, but I'd recommend it only for the truly brave. As for the beancounters, next time I'll return to the Interconti Beach and will ignore their knavish tricks.