Must admit that one was not easy. Thanks for trying, everybody. Don't want you to lose sleep over this riddle, so here we go...
That's the "potato" of an Ipomoea batatas 'Sweet Caroline Green Yellow' (that's not the same as Ipomoea tricolor morning glory). Sorry I don't have a pic of the plant in my garden, but here's a link to a web search for it.
I use this plant every year in combination with some New Guinea Impatiens, in a planter in the shade in summer. Works well. I shall now plant this "potato" in the soil and see it it grows.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.