A few days ago I asked what this was.
According to experts (aka Mum and Dad) it's a beech. Not a pot plant in the slightest but it's going to be one until it gets too big. I'm not going to turn down the opportunity to have another nice green thing in a pot in my garden. And when it outgrows the pot it can have a bigger pot or feed the brown bin, whichever seems most sensible at the time.
Free plants are good! Thank you, Mr Birdie (or whatever imported the seed).