Thanks everyone for your help! Just updating in case anyone has the same problem: it looks as though it's roundworm

I dosed her food up with flubenvet two days ago (just in case) and today her poop had a couple of adult worms in. Pic attached is vile. So strange as they have monthly worming treatment.
I'm going to sterilise the coop, dose the food with extra flubenvet and keep her separated until she's not pooping them out any more.
And resist the urge to burn everything I own.
If anyone has any recommendations of how to avoid this again, I'd love to hear it.