Just to complete this item (sorry it has taken so long!) but it should be noted that the problem was in the cap for the reservoir. The original cap had lost the capacity for completely sealing the system. Replacing the cap it little cost, solved the problem. There was no evidence when inspecting the cap that it was the cause. After months of driving many miles, including towing a caravan, there has been no loss of coolant.
Graham Carling, who posted the original question.
August 2011