Closer observation solves the mystery. All the advice says they won't eat in the first 24 hours and it is no use feeding them, but I added three bags of live daphnia to the tank while the fishes were still floating in their delivery bags, just in case someone felt peckish after their journey. The good thing about live water fleas is that they don't rot if nobody eats them, they just keep on swimming around happily and maybe even making baby waterfleas. Daphnia are attracted by light, and it seems even shy little fishes will dare to venture into the light if the lightest part of the tank is filled with yummy daphnia. I hope this is a sign that they haven't been overly traumatised by their journey.
Now all I have to do is resist the temptation to turn the lights on in the tank so that I can see them better. Give them time to explore their new home in peace. Now that they've filled up on daphnia all the cardinals have found each other and are keeping together in quite a tight little group between two rocks. The rasboras have divided up into two groups, one group is swimming laps at the top of the tank and the other is swimming up and down from top to bottom along the far left corner of the tank. The bristlenose vanished into the undergrowth where it will find plenty of algae to chomp.