Thanks for all the advice. I have cleaned the coop today, can't see evidence of mites anywhere nor on the birds themselves. They slept in the tree again, 15 metres high, 30 ft up so they reasonably safe (our predators are red fox and badger) but obviously I want them back in the coop. There are no mice (we have cats)
I have put a pen around the entrance and three attempts this morning have failed. They tentatively go to the seed outside the coop then run away before I can close the pen. Yet at the back of the coop (no door) they are pecking and scratching all around and going very close. I even tried cat biscuits as they love them but they still got spooked.
Planning to try again every hours in the hope they get the idea but I am not feeling confident. The snow is expected to arrive next weekend and the coop was meant to move to the barn before then but I think it is best not to move it yet. I am concerned they will suffer when the temperature drops