Update - I changed the nut on the underside of the false-bottom for a 1/2" BSP barel, and ground off the last 3-4mm of the barel so it sat nearly flush with the bottom of the boiler.
With this I only get 500ml of liquid left in the bottom of the boiler when I pump out. I don't think I can do much better than that.
With this I only get 500ml of liquid left in the bottom of the boiler when I pump out. I don't think I can do much better than that.