I can't see from the picture but here are a few things to look for if you think it is an apple tree sapling. The leaf stem from the bottom of the leaf to the point it is connected to the stem will look like it has hair, and the leaf will be darker on the top and lighter on the bottom sometimes depending on variety the bottom of the leaf will also be hairy looking or have a peach fuzz look to it. As to what variety you never can tell until you actually have fruit. Do you have any apple trees or crabapple trees nearby?

Yes, thanks Tmh. There are four apple trees of various varieties in the garden - about 40 ft away from this sapling. I don't think they're hairy (bit wet at the moment: I'll check carefully tomorrow), so if not Apple, any other ideas??

They look like wild plum or blackthorn to me. I'd get them out - they don't make good trees, unless you like the wilderness look.....

Thanks Beattie.