Can anyone tell me what this is?

It looks like a Cornus. What a lucky find! It could be one of the deciduous coloured stem types. It may have been sown by a bird or sprouted from a twig stuck in the ground. Or do you remember planting one?

does it have those nice red stems? This is a sizeable shrub, but not overpowering or difficult to control. Very attractive. In the grounds of a new school building near me the landscapers have planted big groups of several different ones, and they look very gd - golden one/variegated leaf one/red stem one. You could get it some friends!