Thanks Walter & Michael,
I got it to work by setting the Video Playback to Blackmagic HDTV 1080i 25Hz – 8 Bit (1920 x 1080) setting;
originally, I had been trying the Blackmagic HDTV 1080i 25Hz – 10 Bit (1920 x 1080) stting, but this played back in realtime WITHOUT any down conversion, so I got 1024×576 pixels out of the 1920×1080.
That shouldn’t happen, should it?
I’ll send the info to Decklink, directly, and see what they say.
(I had posted this on the Decklink forum some days ago, but got no reply.)
Thanks again for helping me sort it out.
Neil.