Ammonia levels are still high (1.5mg/l and at the darker end of the colour band to boot) but the fish seem a lot happier than they were on Friday, when levels were even higher, so I am hopeful that in due course everything will settle down properly.
Meanwhile, in comments the other day, Rob suggested that by conditioning my water to remove chlorine (poisonous to fish) I might be releasing ammonia (also poisoning my fish!) from any chloramine there might be in the tap water.
I’m not sure what the solution would be if I were to find myself on that particular example of Morton’s fork – condition the water and poison my fish with ammonia, or don’t condition it and poison them with chlorine – but fortunately that isn’t my problem. I tested some conditioned water for ammonia and it comes out at nil. Phew!