I have drained my vented central heating system to clear sludge. The header tank has refilled but the system has not. I have removed the pump which was full of debris but there is no water coming out of the pipe from the header tank to the pump (a drop of eight feet). On loosening the compression joint at the base of the header tank pipe, water drains and so does the header tank. I obviously have a blockage between the pump and the curious looking unit that the header tank pipe goes into. This is about 5 inches tall, flat on the bottom and domed on the top. The expansion pipe returns from here back to the header. What is it? Should I remove it to clear the area before the pump?

I'd appreciate some advice.

Is it possible that the device you are talking about is a check valve? It would (should) have an arrow on it somewhere indicating the direction of water through it. If it is, water won't go in the opposite direction.
And how are you refilling the system?