Thanks, Bea, for the 11 March pictures and videos of copulation. On 26 February the male made a mating attempt which was almost instantly and rather angrily declined - the attempt was captured on video and may be watched on the site in which the live cam stream is embedded (it's the compilation video dated 3 March - the males antics start at around 1:45 of the video). Something seems to have changed over the two weeks which separate 26 February and 11 March.
Obviously, I've got some reading to do here,but I can't help wondering: could there be any chance whatsoever that the couple are entertaining the idea of raising another chick in another nest? In a few weeks the chick in this nest will not require round-the-clock care and attention. Were the female to be able to produce another egg (how long is it between copulation and egg-laying?), it could hatch with the current chick not too far from fledging. It takes only one adult at a time to incubate an egg, so perhaps after a new egg is laid, the chick hatched in February could then be supported by the adult not on incubating duty at any given moment...
Or is this idea plain heresy?