Thank you both for your kind help. I checked her every hour from 4.00 until 7.00, and there was little or no change. She was just standing in the same area, looking as if it was painful to walk around (this is a pullet who has been jumping up on to my shoulder when she got the chance.) I tried tempting her with mealworms etc but she didn't want to eat or drink. Crop felt OK, not squishy or compacted, but very little in it. Comb still bright red and erect. Vent clean, no poos observed but there was a runny watery poo on one of the perches in the run, don't know who did it. Eyes and nostrils clear, breathing OK, except that she was clattering her beak open and shut. They've been fed on a new bag of organic pellets and a controlled amount of fresh greens, no long grass etc, with a small helping of mealworms and corn each day (less than an egg cupful.) Run and coop exceptionally clean since the new girls arrived, after my big clean up and disinfectation, so no parasites and as bacteria-free as possible. No attacks or disturbances or bullying. All are commercial hybrids thus fully vaccinated anyway. It looks like peritonitis to me, but I think that's quite unlikely in a POL bird who had laid her first 3 small eggs normally.
At 6.00 I put her in a nestbox - not much point in stressing her out by separating her, I thought, as it doesn't look infectious and anyway they've all been exposed to it. By 7-15 the other three were roosting on the perch and she had moved out on to the floor of the coop.
Yes, Mrs B, this group of 5 POLs arrived a month ago, from a farm I hadn't used before, because my usual trusted breeder, Chalk Hill Poultry, seemed to have gone off the radar and I thought he wouldn't be able to provide any. The new farm looked OK, - they were in a barn, not out at grass, but it was the week after the first lot of snow so I thought this was a good thing for growers raised commercially over the winter. I examined all the birds, and they were all very lively and growing on well to start with. The remaining three seem fine ATM. The first one to die was a RIR hybrid, last week - again, a sudden deterioration, followed by an apparent recovery, and then death overnight in the coop. I supposed that it was just one of those rare occurrences, maybe a heart attack, but now I'm wondering.