Tengu,
I find this an immensely frustrating subject. There's a reserve close to me, an old gravel quarry which was planted up in the 80's. On the margins are old roads, hedges and field systems, some of which appear to be of considerable age with big oaks, beeches, ash, pines and an astonishing variety in the hedges.
Some of the plantings from the 80's are not of native species and there is a vast array of oaks, many of which I've simply given up trying to identify as they don't seem to fit any single variety.
Maybe I'm just being thick but I have wondered about hybridisation, although I can't say I've looked into it. Is it a possibility?