How do I re charge my Worcester Greenstar 24 boiler?

I have bled my radiators, however the bar has dropped now from 1.5 to to 0 on the boiler. There are no instructions in the operating maual for re-filling the tank. I think the rads may still need bleeding, but since the bar dropped, no more air or water is coming out. Perhaps when the bar is raised I can re-bleed them till water appears?

Many thanks.