This is supposed to be Syringa x prestoniae, identified as such by knowledgeable people, though it is described by Hillier very definitely as a shrub, whereas this is emphatically a tree, albeit a little one. S. x prestoniae are an extremely hardy race of hybrids, first raised in Canada in the 1920s. There is a number of individual varieties, but no clues as to which one this might be.
It does not have the familiar scent of lilac, in fact it has no scent at all, and the flowers are hardly extravagant, but rather subdued. It does have a good shape with a canopy that provides a nice dappled shade for plants that like it, like Alchemilla mollis, Euphorbia (various) and Geranium macrorrhizum.