The leak is from a nipple fitting in this pipe junction. It is located right behind the boiler. Two return lines join here and feed the condensed water back into the boiler.
Today, May 15th 2019, Steve Dodd made a temporary repair to the leak. He said when the outside temperature warms up and the boiler can be turned off, he will come back and replace the leaking part as part of the permanent repair.
Art Livesey