Gremlins follow up
I have now had the time to put the layout up again at home and guess what; the fault has gone!!. I checked through all the wiring and connections with no problems. A useful exercise as I managed to remove some duplicate wiring and rewire one area which hadn’t functioned as I planned. I then tried to re-construct the fault I had had. I set the points as they had been and then systematically started to short out section gaps until hey presto I got the fault. It was all down to one narrow gap that had obviously closed up with the heat of the hall. Oh the joys of exhibiting. I have now enlarged the gap and hopefully will have no problem next time.