Just to complete the story, I received a telephone call yesterday afternoon from Rosie, a very pleasant lady at Southern Water. She explained that the cause of the problem was likely to be the pressure from the pump being too high. SW have now fitted a pressure monitor to keep an eye on the matter. Any further problems and I just have to call Rosie.
She also assured me that the problems I had been experiencing were not harmful and I just need to leave the water to stand for a short period in order to let the comprssed air in the water escape.
Bill