no hot water to storage tank?

potterton free standing boilor ,feed to hot water tank will not get hot, heating working perfectly its fully pumped ,from boler gravity fed to hot tank even put bleed valve onto it as there was not one ,still cannot get hot water to flow around coil in tank and back to boiler ,also there was no header tank or breather pipe fitted so we fitted one ,also even a supply feed from new header tank to get water in as we could not find one.yet this heating has run for 6 years it must been filled fromthe bleed off valve? any ideas as iv,e ran out .and ran off