I pulled the spark plug cables and found water round the spark plugs. Probably from too enthusiastically washing.
After drying up I found the rough running still remained but to a lesser degree. A new inspection revealed lots of rust round the spark plugs closes to the bulkhead, could hardly see the hex head!
After scratching and vacuum cleaning round the spark plugs I also cleaned the spark plug cables which were covered in rust.
New test drive today, problem gone.
A cover over the camshaft cover and spark plug would probably be of great benefit.
