If it has a woody stem then it is probably a willow. You live in Ayreshire, Mf, then the shrub was probably bought from a nursery and it seeded into the compost, not at all unusual here in Scotland. It will grow into a slightly stragley, mid-sized shrub that is not unattractive.

Because of the shape of the leaves and the red-blush to them I would vote for a Bilberry (Vaccinium myrtillus) seedling, especially if the nursery it came from was near acidic-soiled moors or hills.
Until something flowers it is hard to say as foliage can be near identical between plants that are completely unrelated, whereas flowers can tell you much more.

Many thanks to both of you.
As you say, Buddlejagar, it's very difficult to know what you have for sure unless it flowers, and I look forward to seeing what it produces. A Bilberry would be quite exciting!
However, I'm inclined to think it could be a willow, as Bulbaholic suggests. The reason I'm saying this is that I have a Woolly Willow, and while I know it's not that, it does however, have very similar shaped leaves, and the arrangement of the leaves around the stems seems almost identical. It'll be quite exciting to see what develops. Thanks.