Okay I found the small tank (quite grotty and brown in there) and the 15mm pipe which leads out to the larger pipe (then tees off to the pump)
I banged it hard and even tried a plastic electricians draw wire down the pipe entrance from the tank, to see if any blickages could be felt but it went clean through and then after about 7 ft of draw wire it stopped dead. I looked in the airing cupboard where the 15mm pipe joins the larger one and it its a tee junction. So this may account for the stop dead draw wire. But still the problem persists and the blockage is not resolved. Any other ideas please?
Just one last thing before I accept the answer if you don't mind.
I have a company coming round on Friday to change the pipe where the blockage is (he checked it with a magnet and suspects a problem on the next joint of the 22mm pipe before the pump). I also asked him about the poor water quality and sludge in the feed tank and he recomended a powerflush to the whole system (11 radiators) at £350. Do you recommend this and do you feel it is necessary on the information you have(as it is quite expensive)?
Thanks again for your help