Radiators aren't very expensive and you can do the work yourself.
To replace:-
With my open-vent system, I go in to the tank and tie up the ball in the heating system expansion tank, then go down to my bathroom, attach a hose from the heating system drain plug and lay the other end in the bath, open the drain plug and drain the system down. If your radiators upstairs, you won't need to drain it all, but if you're not sure let it all drain out.
Then just put rags down beneath the radiator that you are wanting to remove, and use a spanner/monkey wrench to undo the nuts on either side of the radiator, and just simply lift off.
Measure the radiator and the connections, and then go to Homebase/B&Q/PlumbWorld etc. and purchase a new radiator and some central heating inhibitor (Fernox are the best known brand).
Re-assemble. Then close heating system drain plug in bathroom, go back up to the loft, untie the ball in the heating expansion tank, and your system will start to refill. Add the central heating system inhibitor at this stage. Once the system has stopped refilling then go around your radiators 1 by 1 letting any trapped air in them out by using an alun key with the valve at the top of the radiator.