Thanks all. Will email you Matt. And it acts like it could be blocked, so i'll try that filter on top before i remove the top itself; plus will check the manifold pipe.
So have checked the above, there is no filter (anymore!), but reasonably clean inside; and manifold pipe is sucking well. BUT if i understand the operation correctly, the air valve is not opening when the top tank is full, therefore not dropping the fuel down. It rattles up and down happily enough but doesn't move whether the float is up or down. These are the two float positions...
Any thoughts on what might have happened ? I can't see what is broken or missing, it looks fine, as if it's always been like that. But if i blow through the air tube nothing comes through in either float position (unless i manually move the valve).