Garlic mustard is the people’s choice for that plant, IF the leaves smell of garlic. I’m not going to the allotment for a few days, so won’t be able to check that out. Leopard’s Bane has also been suggested, and ‘money plant’. And, yes, it does have a look of a strawberry plant about it, which was another suggestion.
What happened was, whilst I was in Libya during the summer, The Old Man dug a trench to lay path edging, and the mystery plant appeared in the heap of earth thrown up from the trench, along with the comfrey, a few weeks later. The allotments are a century old or more, and we’re theorising that my predecessorswould have grown herbs, and that plant, and the comfrey, had lain buried and dormant for a long time, before getting a chance to re-emerge.
I’ll keep it safe until next year anyhow, and see what flowers or fruit it puts out.
Thanks to the several helpful virtual horticulturalists who’ve posted suggestions.