Very helpful Tony, thanks! I will have to see if I have any brake cleaner about somewhere!

Steve, it sounds very much to me like you could be the winner. Perhaps the jets are working loose or something then - I will have a good check tomorrow morning on that front. The float chambers were half full when I removed the float itself, I don't know but perhaps were completely full with the float in?! I will do more checks

I'm really enjoying this troubleshooting, thanks!