Close the Rad valves either end, loosen Rad air vent at the top, then the nuts holding the valves onto the Rad, drain down the Rad. Remove the Rad, take it outside and with the Rad on its end (one of the valve holes on the ground) stick a hosepipe in the top hole and give it a good flush until it runs clear. Refit the Rad, leave the air vent open and gently open one of the valves until the Rad is full, close the air vent, open the valves. Should get fid of all the crap.
no, dont use a chemical if your going to hose out the gunk from the rad. the chemicals dont have enough time to dislodge the problem when you hose a rad.
if taking the rad off the wall is too much trouble you should use something like Fernox F3. drain a litre or so out your system and add the product through either a rad or the filling loop under or close to your boiler. let the system work for about a week (up to 2 if you have 8mm pipe anywhere in your system) then drain, fill, drain and fill again until all the product is has been removed. fill the system up to 1.5 bar and you should be good to go.
most companies charge around £200 for this!!!!