View the thread, titled "Expansion tank overflow now that new condensing boiler has been fitted." which is posted in Boiler Advice Forum on UK Plumbers Forums.
Is your pump correctly wired? Your boiler might have pump over run build in but if it is incorrectly wired it wont work. Look at installation instructions for pump wiring. Might work to make open vent pipe higher as no pump over run on your boiler.
Official Sponsors of Plumbers Talk
Reply to the thread, titled "Expansion tank overflow now that new condensing boiler has been fitted." which is posted in Boiler Advice Forum on Plumbers Forums.