Expansion Tank won't drain?

Our central heating system stopped working - the boiler would remain on for a few minutes but then cut out, all radiators remained cold. Before this we could get some radiators to work but not all at once - i.e. turned the upstairs ones on to get the ground floor radiators to come on. We drained the system, stopped the water coming into the expansion tank and attached a hose to the radiator valve and black sludgy water came out. However the expansion tank has not drained or gone down at all.

We tried a pipe cleaner to check for blockages which went down about 6ft. We tried forcing cold water back up the system which came out into the expansion tank through the expansion pipe.

We don't know what else to try. Anyone have any ideas - or do we need to call in the professionals £?£?£?£?

Thanks