Timothy Murphy wrote:
though whether it is the lost stopcock I'm not sure.

If the aim of the exercise is to isolate your incoming mains, it might
be more sensible to hire a freezer and fit a stopcock in your property.
You can also, I believe, isolate MDPE plastic by squashing it with a
clamp. I think you can only do this once at one point. I haven't dome
it, so you'll have to research the manufacturers' web sites (Durapipe,
Georg Fischer) or pick the water suppliers' collective brains.