One of my recent endeavours is taking up a distance learning course in horticulture (RHS Level 2 Certificate). Here I am, playing the botanist, and attempting to identify one of the plants in my garden.

I have now decided is almost certainly not Eupatorium purpureum, which is what I had it down as — that has whorls of leaves at each node and this has alternate leaves. Any ideas? I’m leaning towards a fairly standard Spirea japonica but would love to be able to identify the cultivar too.