regarding the tank in loft the vent pipe is right on top of overflow pipe outlet
and judging by the tide mark on the ball the water was going up the vent and splashing on the overflow pipe
Depending on type of valve and problem and your skill level it could be DIY able. May be possible to override the valve motor to get heating as a temporary measure. Is the hot water heating up ok using the boiler? (Not an immersion!)
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