Apologies to all who offered assistance. My Saturday away turned into a very long weekend through unfortunate circumstances and I was unable to get back to the problem. However my son was at home most of the time and monitored the situation. The calls continued until Monday and then over the next two days petered out to irregular intervals - sometimes 2-3 followed by a gap of 2-3 hours. However I am glad to report that they ceased altogether about 48 hours ago. May well remsain one of lifes unsolved mysteries. Thanks again for your help.