Fred wrote:

The weird return stop valves fitted by plumber (UK) to comply with the
law (UK - he insisted on this) did not help matters. Maybe you have
something like this as well

He may have been right... if the tap was a genuine mixer tap (i.e. it
mixed hot and cold in the tap and then delivered warm water out of the
one single spout), then you need the double check valve. If the tap was
like most mixers these days, where there are two concentric spouts, the
mixing (such as it is) happens outside the tap. Hence no need for the valve.

--
Cheers,

John.

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