I've been told that this is.....

....buckwheat.  If it is, how on earth did it get there.  I suppose a passing bird might have dropped a seed.  There's only the one plant which is getting taller and taller.  Thought I'd leave it and see what happens.  The extra is something I made for lunch today from a recipe given to me by a friend.
They're called Kolokithokeftedes (pronounce that after a glass or two!) & were delicious.  One I'll definitely be making again.