Thank you for all your replies. In answer to the various questions:
Yup - perch not nest, dust bathe and cleaned out every week
Pop hole fine
Did have a few mites earlier in the summer but nuked them with appropriate gunk and all OK now - this is the only chemicals they have had contact with
They get Poultry Spice and Battles Poultry Drink regularly
Rest of flock OK - moulting but no Cherie Blaire impressions (Peter you are definately a bit strange

)
I did speak to the nice lady at The Domestic Fowl Trust and she repeated a lot of your advice and then said sometimes young hens in their first moult do go to extremes, so as long as they were happy and healthy, not to panic.
I also asked her about my Victor\Victoria bantam - I put a post on a while ago as I had a hen who had suddenly started crowing each morning. She said - well dear, it will be hormonal and since you can't get HRT for chickens I wouldn't worry about that either
Anyway I reckon a Clint Eastwood style poncho is going to be the answer. No probs with the lack of shoulders and free access for egg laying and pooping. You heard it here first - patents pending
Sue