No to the copper wiring. It’s electrical cables from when there was a summer house at the back that was wired up (I think). The black stuff of just from the cover that I knocked out. I came apart like slate. That white block though looks odd. Can ups it be a fire brick? What does asbestos look like?
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If so, my money is on it being a still!
https://forum.gardenersworld.com/discussion/1010384/how-to-upload-a-picture-or-embed-other-media/p1
My rhubarb has outgrown its forcer ................
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.