That looks like a seedling of the Pheasant Berry, Leycesteria formosa. There must be one growing nearby, maybe in a neighbours garden.
It's quite a nice ornamental shrub, but gets very large very quickly so you may want to dig it up. Sadly, it is not edible to tortoises.
http://m.thetortoisetable.org.uk/m/p...main&catID=484