hot water venting into expansion tank

I posted a query just over a month ago about hot water venting into the expansion tank. I had some useful answers, causes could be a hole in the heating coil in the tank or a blockage or partial blockage caused by deposits of iron oxide probably at the T junction where the feed pipe joins the horizontal pipe that goes from the pump to the expansion pipe in my airing cupboard. I tried the magnet test and it stuck well to the piping in that area.
In case it was a partial blockage it was suggested that I drain a few litres of water off, add some X400, top up and leave to circulate for a month. This has had no effect so am assuming that there is a total blockage in a part of the pipe as I was told that the sludge remover would not work on a completeley blocked pipe. I suspect that the blocked bit is from the T junction to the upward expansion pipe which is about 13 cm from the T junction, the down pipe from this goes down through the floor somewhere. The magnet still sticks well to this section of pipe. The pipe section from the T junction of the feed pipe to the pump, which is about 19cm away must be open to some extent even though the magnet sticks here as well because the level of water in the expansion tank does not alter so the same amount of water must be going down the feed pipe as comes in from the expansion tank and so must be getting to the pump.
Does this sound right? I was informed before that it would be a good idea to drain the system, have to anyway now after X400, and get the T junction replaced. I have arranged for a plumber to come and look at the problem as I don't have the skills to do it.
Would I be right in assuming that the problem is not a hole in the coil as that would force extra water into the expansion tank and cause it to overflow? I have turned the feed pipe from the main cold water tank to the hot water tank off to see if the hot water venting into the expansion tank stopped, it didn't. If there was a hole in the coil would the pressure of the cold water tank contents force water into the heating coil? If so then surely turning off the supply would stop the flow into the expansion tank, or would it?
I hope it is not the coil as one cannot get at the pipe connections to the tank as some are at the rear and it is impossible to get at them. When they built the house 13 yrs ago they must have put the tank and pipes in and then put the walls in after. The only way anyone could access these connections would be to cut a hole in the wall at the back which is part of the wall to the 'boxroom' so you can see why I am really hoping that it isn't this that is the cause of the trouble.
Sorry this post is so long, I hope some one can reply with some useful ideas as they did before.

certainly sounds like a blockage. have u not tried to drain down the system remove the pipe that the magnet is sticking to? the cyclinder would more than likely have had the connections fitted with the pipe soldered then pushed into place then working off the pipes that how i do it. if you understand. if there are any compression fittings off the pump and valves or valve then cut the feed and expansion pipe and reconnect with compression in u dont have any soldering equipment. then get the pushfit cap ends from a plumbers merchants and cap the feed andexpansion then get some mains cold thru it.

Thanks for advice, actual plumbing a bit beyond me but what do you think of this? You will probable think I am crazy! I decided to try and put some wire down the feed pipe to see if it could get to the blockage. I tried 3 different wire types as the problem was getting it to turn ninety degrees at the T junction. Did manage it and may have partially cleared the blockage as the hot water has stopped venting into tank except when pump turned up from medium position (where it normally is) to the high position, the hot water vents into tank again as if the flow is too high for the partial clearance of the blockage, if indeed that is what has happened.
My next question is - how long can I leave the x400 in the system to see if it will act on this sludge now, hopefully, it can pass along the pipe? The X400 has been in a month and the water in the tank looks like rusty soup. Or should I get the system drained and get the affected section of pipe changed, though not keen during this cold spell.

I'd get it flushed out now - empty the header tank with a bucket to prevent it re-blocking if you have managed to make an opening in the pipe.

If you can leave the drain valve open and adjust the ball valve in the tank to enable a constant flow through the sysemt, thus clearing out as much as possible before using more sludge remover, then so much the better.

Thanks for advice, so you are saying to drain the system, flush it through as much as possible and then put another lot of sludge remover, say X400 again, and leave that to circulate again. How long do you think I should leave it this time?
As regards draining there isn't what I would call a proper drain tap in the lowest part of the system, there is one situated above the boiler that a hose pipe can be connected to to drain the upper floor, but each radiator downstairs has a little drain tap on which you have to unscrew a type of nut. I did drain some out to put the X400 in and it took ages. I presume this is the only way to do it though.