I modified the pushrod to incorporate a spring link:
It seems to work fine now. When the pump is priming itself the spring hardly deflects and full piston travel ensures rapid priming. When coolant reaches the delivery NRV at the top of the pipe pressure rises and the spring deflects by about 1cm thus reducing piston stroke. Now I get a good coolant flow and the little cups don't overflow.
When I'm happy with the design I'll probably loose that cheap galv Screwfix threaded bar and use something that makes it look a little less codged together!
I'd still love to take a look at the genuine Wicksteed arrangement though - maybe I've improved upon the design?