I woke up in the middle of the night, and decided to shine a torch in on the shrimps to catch them unawares. They were all swimming about, eating, climbing the plants, etc, even including the really, really bad one. It had regained some of its colour, too. Then, after about 10 seconds exposed to the torch light, it started twitching on its back again, and the others all went into hiding! I could only see one this morning, one of the original healthy ones. Why would their behaviour, particularly that of the sick one, change so suddenly?
It's often the cas hat Cherry shimp will hide in th day and only really come out at night, especially when first settling into a new tank. They don't like bright light so I expect the torch in th night spooked them.