Boilermate cold feed blocked from F & E

I had to drain the system down to move a radiator, while the system was empty I took the chance to add an Adey MagnaClean Pro 2.

On trying to refill the system I notice that the header tank contents does not fill the main system.

I have emptied and cleaned the header tank, removed the side inspection/immersion cover from the main system and fully emptied and cleaned what I can out by filling from hose and syphoning out contents.  I have pushed a flexible curtain rail to clear any blockage, it manages to go down to a depth of 1.58m until is seems to hit something solid. I have also tried to force mains water pressure down the cold feed, tried to attached a bicycle pump (not successfully).

What I would like to know, how is this cold feed routed from the header tank to the main system, where does it come into the main system, is it the 22mm vertical pipe roughly in the center, that looks to hit the bottom of the tank with what looks like some folded copper at the bottom (hard to see as the is a horizontal return from radiators pipe obscuring my view). I would have thought it would enter the main system at the side vertically from where it left the header tank and be 15mm but I just cannot see where.

I have filled the system via the drain cock as others have suggested and added some cleaner to the system to see if this helps in any way, what I can say is after 15 mins of circulation, there is already a lot of sludge caught by the MagnaClean (which I was already fitting to stop this type of problem).

Any help appreciated.

Okay after a week of leaving the system running with some cleaner that I added last weekend, it was time to drain and flush out the system. The cold feed is still blocked, so off to get an inspection camera and see where this cold feed enters the system. Basically it enters the system somewhere  between 300-400mm from the bottom of the tank directly below where in exits from the header tank. It's just out of sight by looking directly through the immersion aperture., by using the inspection camera to locate and direct my flexible cleaning spring from the DIY shop I prodded around inside the 15mm pipe, still no luck with water flow but a slight trickle could be seen. I tried again from the header tank and hey presto water began to flow, slow at first but now flows very well.

Hope this helps any others out, it basically is possible but quite a pain, I certainly recommend fitting a MagnaClean, the muck that was in the system was probably going to cause problems in the future, as I understand this will reduce those chances.