On the HSS walk to Sun Wood today, an Elder with linear cut leaves was noticed on the woodland edge .
It seems to be a combination of Sambucus nigra var. 'Laciniata' (Parsley -Leaved Elder) and 'Linearis'; not quite fitting the description of either. Laciniata has been known since the 16th century and does occur occasionally in the wild.
I suspect it is quite a rare tree, even in local gardens. Note the bifid and trifid, as well as entire linear leaves.