After working through the carbs and compression tests and manifold exhaust leaks the problem was solved adjusting the exhaust tappets. The manual suggest 0.12mm gap. There was hardly any play in the tappets so set them at between 0.15-0.20mm. It now idles perfectly and runs like a dream. Airscrew is set at 1 1/4 turn out. Advice was provided by a 2CV expert who suggested that with modern fuel and running in hot weather old 2CV's are now run with 0.25mm gaps whereas the manual suggests 0.20mm. Hope this helps others with the same problem.