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Boiling noisy electric Hot Water Cylinder

Hello, my low pressure electric hot water cylinder is rumbling and way too hot. Overflowing onto roof and very loud. Have turned it off at the mains, then at the wall inside cylinder cupboard. I'm not sure what it needs, a new element/thermostat, a drain of sediment or something else. Would really appreciate assistance asap as have 4 children and 3 adults in need of daily hot water. Thank you