I pulled it from a crack in some concrete. There didn't seem to be any parent tree nearby.

Catalpa? In England?
Catalpa seeds do germinate readily in North America
Northern Catalpa (Catalpa speciosa)

They do grow in England.

DaisyI said:They do grow in England.

Yes..normally Catalpha biginoides.but normally just in botanical gardens, parks, stately homes...just occasionally in private gardens.
Leaves in 3.s. not sure it is Catalpa.
This is one in our old garden ...Catalpha biginoides aurea.

Apparently, there's at least one Catalpa speciosa growing in Cambridge. The RHS lists 3 nurseries in southern England where you can buy one.

The description says Catalpa leaves grow in opposites or whorls. But telling whether is a speciosa or a bignonioides might be trickier.

https://www.carolinanature.com...).

Yes I thought it was a Catalpa. That's why I pulled it out of the pavement. But I'm fairly familiar with Catalpa and now I'm not sure it is. The leaves seem too waxy and I recall Catalpa having three buds at each node which this one doesn't have. Not sure though. I don't have an actual Catalpa nearby to compare it with. I'm wondering if alternatively it could be a Populus of some kind?

https://bwwellsassociation.wor...

https://www.google.com/search?...

Thank for all your input on this. I just had another look and am more convinced now that is a Catalpa. Seedlings might be different with only 2 buds at the nodes. Definitely not a Populus because the buds are ( almost) opposite. Leaves don't seem as waxy as when I first pulled it up last summer ( tiny little thing then). Also there is a Catalpa growing in a park about 100 meters away from where I found it. Many many thanks,

Catalpa have 2 opposite leaves or 3 leaves in a whorl.