Re the above thanks all for your feedback. I am pleased to say that a resourceful neighbour of mine resolved the problem. I do not know the details yet but I have heard that she netted the signets and relocated them to a point above the weir. I was absolutely amazed when the all turned up at the nest which is just below my balcony. I was further amazed when later the 5th (missing) signet turned up. She must have gone looking for it and found it. Anyway the family is doing well and with a bit of luck they will keep away from the weir.