Bet it isn't
this one, which actually has directions from a resevoir (no - really!)
There's one in Caerwent, Wales, which is just down the road from a resevoir, and also fairly close to Caoel-y-Ffin, don't you know?
PSD
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This is the work of an Italian narco-anarchic collective. Don't bother insulting them, they can't read English anyway.