I took a photo of this a few weeks ago because I didn't recognise it. I expect to recognise trees - at least, trees growing in a context that says 'I am a native tree!' like this one. Later, I found the photo and thought 'what was that tree anyway?' So I typed things vaguely into Google, but had no luck. Then the word 'Alder' came into my head, and then the word 'buckthorn'. And so it appears to be. Yet I would have sworn I did not know that tree.
Bad brain, no biscuit.