I'm still waiting on the gasket but swapped the float valve to that from the equivalent Mariner carb (the pin is a bit thicker and machined vs a simple thin pin) - it started on the briefest touch of the key (and no water leak this time...)
My theory is that the simple nature of the pin of the float valve made it prone to being pushed to one side as the float touched it and as a result the valve got a bit stuck.
"Can ye model it? For if ye can, ye understand it, and if ye canna, ye dinna!" - Lord kelvin