My first thoughts are an Ash seedling but something niggling me that it could be something different !!!!!

I was hoping it was going to flower :( Think you might be right, I will have to try and move it.

Not sure, but it looks like a walnut tree to me. They grow everywhere in our garden!

is the new growth red on a walnut tree?

Darkish red on mine

Hmm--the leaves are in pairs, and walnut leaves come singly. I vote for an Elder seedling, possibly with some heritage from a black variety.

100% not an elder. My other choices were...

Koelrutea paniculata or
Kolkwitzia amabilis

How about a Rowan (mountain ash)?

i agree with Steragram a sorbus, probably one of the common red ones grown along streets and avenues. They easily self seed with the help of the wild bird population. We have loads of these saplings that we have to remove in our woods. But then the one I would love to propagate is the pink variety growing in one of my thick boundary hedges. But then it's too tall to reach for cuttings or seeds.

Koelreuteria leaves are alternate, not opposite, and so also are Sorbus. Kolkwitzia are opposite, but they are simple, not pinnately compound. If it's not an Elder (Sambucus) then it is probably an Ash (Fraxinus). A good close-up of where the leaves join the stem should settle it.

Sorbus aucuparia, sorbus domestica and and sorbus hupehensis leaves are surely opposite Tug - sorry to argue. I'm pretty sure rowans usually have more pairs of leaves to the stem than fraxinus too. This doesn't look like the ash we have round here. Would Kolkwitzia self set in the UK? I thought it was grown from cuttings here.

Umm, Steragram, a point of terminology--in Sorbus species, the leaflets are opposite, but the leaves are alternate.

I stand corrected! Ash then.