Could be a tree seedling, can't really make out the shape of the leaves properly - what trees do you have nearby? Does it match any of them?

It's an Ash tree seedling, but not sure of what species.

It's not like what we know as ash - that has compound leaves with pairs of leaflets along the midrib and one at the end.

Sedlings frequently have simple leaves for the first several pairs before they start producing compound ones. I noticed that the newest couple of pairs are becoming compound. If it were here in Arizona, I would say that it is a Shamel Ash (Fraxinus uhdei) seedling, but I bet that doesn't grow in the UK! : )

We have ash trees around here and I'm always pulling up seedlings. They have compound leaves when they're only 6 inches tall. I'm sure it's something else.