Thanks for the responses all.
Just to update this, I've done a bit more digging today, right up to the boundary and uncovered this:
The street stopcock is on the other side of the fence panel on the right.
The 15mm copper from the house joins onto.... lead pipe? That's what it looks like to me. I can gouge it with a craft knife and it's definitely not magnetic.
Hopefully that means I can get it replaced for free. Not every day you find over £2k buried in your garden eh?
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And we’ll done for not chopping through it wth your digging tools