Thanks to Bab and JohnD. I think we'll survive until the world's back in order. The house is pretty old so I'll have a go at replacing the valve when rescue might come if it goes pear shaped. I'm not quite sure why the thing doesn't work though: the float is set to cut off below the outflow -...
A novice here: just returned from Hols to discover water jetting out of overflow. After pondering where and why - its the cold water header tank float valve. It didn't cut off - so I consulted DIY book and identified need for new washer. After some time I installed this - and indeed the flow is...