Any idea please?

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It can't have appeared suddenly, it's over a metre tall

And I don't know what it is

I'm not sure either, my first thought was a hazel tree, planted by a dear squirrel. It's going to grow big though. Can you pull it up?

Prunus sucker or seedling. It’s not a hazel, leaf shape is wrong, and hazels don’t have that brownish colouring at the tip.

No room, no room. I hope I won't offend the squirrel by pulling it up as soon as I get home. Thank you