Well, at last I've managed to find a few minutes to look at the front derailleur ( I think the rear just needs adjusting). I undid the cable from the gear and held the cable in my hand. Using the lever the cable retracted into the lever with each action. Each click of the button let a small amount of cable out of the lever. These were quite positive actions, so I assume that the lever action is correct. However, after clamping the cable back on to the gear the original problem occurred. The lever only clicked once when I moved it and the gear went from the small ring straight to the largest ring. Clicking the button it dropped back to the small ring. There did seem as if there was a tiny hesitation, almost as if it thought about stopping at the middle ring each way - but perhaps it was wishful thinking.....
It seems to me that the lever/button mechanism works correctly with no load, but when a load is applied something is slipping in the middle position.
All I want now is for someone to come up with a brilliantly simple solution ! I never like putting pressure on this forum ;-)