I finally managed to stop the hen being broody. Well to be honest I don’t know if it stopped naturally or it was what I did that stopped it.
For about three weeks I had been taking the broody hen out of the nesting box in the morning and putting her the other side of the fence for most of the day so she could not go back in. But of course every evening when I let her back in she raced straight back into the house and sat broody once again.
I assume that every evening this was undoing any good i was doing during the day so after about three weeks i decided to modify the house to add a lock out panel for the nesting boxes every evening.
After 2 days and 2 nights of doing this she is back to her normal self so I think it might have done the trick but as I said earlier maybe she would have snapped out of it anyway. When it happens again I will get a better idea of it’s effectiveness.