My downstairs radiators don\\\'t work but upstairs ones do. I understand this may be because the pump has failing so I have to \\\"Turn the central heating system off, wait for the pump to cool and try to start the pump manually. In the middle of the pump, remove the screw and turn the pump\\\'s manual starter - the impeller\\\" Before I do this do I need to turn off the electricity supply to the boiler and pump? Also when they say remove the screw is this the big silver thing on the front - only it doesn\\\'t seem to turn.....