Looking at picture http://allyoucanupload.webshots.com/v/2001173143539993472 there are two pipes joining onto the large one coming out of the bottom of the pipe.
I think the smaller one is the 15mm Feed pipe from the F&E (small tank) in the loft
I think the larger one is the 22mm Vent pipe which curves over the top of the F&E tank and will vent steam or water in the event of a blockage or overheating.
The blockage will almost certainly be in the T where the 15mm F&E pipe joins the pipe from the pump.
If you hold a magnet against it, you can expect to find that the magnet is attracted to the coper pipe at that point. this is because the black sediment if from corrosion of the iron in your radiators.
You have to cut out this joint, and either poke it clean or fit a new piece. It is difficult to solder or unsolder wet pipes so you will probably find it easier to remake the joint with compression brass fittings. these are moe expensive but you will only be using a few. You can also take compression joints apart in future if it ever blocks again.
As for the pump, there are valves on the pipes going into it. If these are not jammed, you can close them and undo the pump connectors, your pump looks quite modern so you should find a replacement pump of the same type fits straight in.
In your case, you can drain the upper part of the system (if it is not already dry) so you can take the pump off even if the valves are seized. If they are seized, put new valves on while you are doing this job ( a pump has moving parts so needs occasional replacement).
Also, buy a litre of Sentinel X400, which is a cleaning chemical that will loosen remaining black sludge and sediment so that it can be drained away, and Sentinel X100 which is a corrosion inhibitor to prevent it happening again. Do not add the chemicals until you have repaired and tested the system so that you do not have to drain it out again.
If you can possibly afford it, and if, as I think, the cause is black sediment, also buy a Magnaclean which is a device that traps and holds circulating black particles and prevents then accumulating into a blockage. It will cost you £100 or less.
There are also chemicals made by Fernox which is another excellent brand. Cheaper ones may not be as good.
I think the smaller one is the 15mm Feed pipe from the F&E (small tank) in the loft
I think the larger one is the 22mm Vent pipe which curves over the top of the F&E tank and will vent steam or water in the event of a blockage or overheating.
The blockage will almost certainly be in the T where the 15mm F&E pipe joins the pipe from the pump.
If you hold a magnet against it, you can expect to find that the magnet is attracted to the coper pipe at that point. this is because the black sediment if from corrosion of the iron in your radiators.
You have to cut out this joint, and either poke it clean or fit a new piece. It is difficult to solder or unsolder wet pipes so you will probably find it easier to remake the joint with compression brass fittings. these are moe expensive but you will only be using a few. You can also take compression joints apart in future if it ever blocks again.
As for the pump, there are valves on the pipes going into it. If these are not jammed, you can close them and undo the pump connectors, your pump looks quite modern so you should find a replacement pump of the same type fits straight in.
In your case, you can drain the upper part of the system (if it is not already dry) so you can take the pump off even if the valves are seized. If they are seized, put new valves on while you are doing this job ( a pump has moving parts so needs occasional replacement).
Also, buy a litre of Sentinel X400, which is a cleaning chemical that will loosen remaining black sludge and sediment so that it can be drained away, and Sentinel X100 which is a corrosion inhibitor to prevent it happening again. Do not add the chemicals until you have repaired and tested the system so that you do not have to drain it out again.
If you can possibly afford it, and if, as I think, the cause is black sediment, also buy a Magnaclean which is a device that traps and holds circulating black particles and prevents then accumulating into a blockage. It will cost you £100 or less.
There are also chemicals made by Fernox which is another excellent brand. Cheaper ones may not be as good.
