I've got nothing to loose by trying it so I turned the clutch pulley down as small as I dared today. Not sure what it was made from but it was a bit of a struggle on my little lathe.
The before and after. I've ended up with the smaller of the two pulleys I found for sale, if they had any in stock. It was 128mm OD and is now about 110mm. If I do decide to get a bigger pulley for the gearbox it won't need to be quite as big now.
Todays other modification, or rather quick lash up to see how it worked was to fit a foot throttle. I've seen people have fitted bike brake levers so for now I have done similar using an old racer lever I straightened up a bit. The hole in the bit of wood isn't a clever design feature it just happened to be lying about. The good thing is it actually allows more gentle take offs so I will give it a try tomorrow to see if it's any more driveable now.