sound like u need to turn of the boiler and go to the lowest radiator in the house and undo the valve on the side. Any one will do. It will be messy so lots of bowls and towels. U can control the pressure of how much water comes out however by having someone on the highest radiator with a rad key adjusting how much air gets into the system.
Oh and fit a drain valve when this is done!
PS - i find a water hoover really handy for no mess. u can hire them really cheap
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