At long last the specialist sent me for am MRI scan, and low and behold they found "sub pleural fibrosis" and promptly put me on a hefty course of steroids, starting with 50 mgs of Prednisolone for 10 days then gradually reducing the dose, I am now on 2 mgs per day with a Qvar inhaler taken twice a day with a view to soon dropping the Prednisolone altogether, but to continue using the inhaler permanently.
I have not coughed since starting the treatment some months ago, the relief is immeasurable.