New float valves and seats fitted, carbs back on the bike, and it now runs. but only on three cylinders. The dead cylinders carb has fuel in its float bowl at the correct height, while the spark is ok too. As it ran on 3 cylinders, I squirted fuel with a syringe down the carb and that cylinder fired up for a few seconds until the fuel ran out.
Today removed the carbs yet again, stripped that carb and put it in the ultrasonic bath for another 30 minutes. Couldn't see any problems so rebuilt the carb, and will try again over the weekend. Though I'm not expecting too much..