Thank you, and I know you are right on contacting the vets. The problem is that it is a bank holiday here today so most of them are only open for emergencies, and also not familiar with chickens.
I have tried the veterinary diagnostics but the phone lines don’t go through, so I’ll get back on google to find somewhere else.
My small flock (of 6 now) does free range in the garden so there is a possibility of wild birds bringing something in, but we have not made any changes to the flock for nearly a year.
I thought I did smell an unpleasant odour from the recently sick one, but my mum can’t smell it, so maybe I am imagining. I’ll look up mycoplasma MG in case maybe it’s that.
Thank you again for your help.