Albion mainsflow contractor

Hi,
i am having som issues refilling an Albion mainsflow contractor thermal store

http://www.advancedwater.co.uk/STOMEMO/A012-088-1603/Albion Mainsflow Installation Guide.pdf

This weekend i drained the system to fix a pipe that accidentally got a screw installed into it. The pipe fix was pretty straight forward. Then i tried re filling the system.

I turned on the water at the mains and the feed tank on the top started fill but only a trickle so i assume it wasn't totally empty. It seems that no water is getting into the thermal store. In the feed tank there is a 22mm pipe out of the bottom that goes into the thermal store.

When i ran the pump it was obvioud there was no water being pumped through the boiler and into the store. As a temporary fix i filled the system backwards from a radiator downstairs. that seems to have partially filled the store as all the radiators are working and hot water, however the water sounds as though it is running into the store and it is still not full.

I am guessing the internal cold feed from the feed tank to the store is blocked. My problem is i don't know how to un block it as it is inside the tank.

Any ideas ???