Well, I am still stumped by this tree, but I have taken careful note of the suggestions, and I am going back there later this week (weather permitting) with a list of things to look at, closely, and if I can isolate any point of note, I'll come back and present my findings.

Nick, I had no idea Davidia was easy to germinate: I've tried and failed several times - maybe the seeds knew that they had been "half inched" from an arboretum *guilty blush* and failed to germinate to teach me a lesson. This mystery tree is certainly not in "particularly open soil" it's on a steep bank, well compressed, hemmed in by brick walls and the oil tank.

Sacha - yes, if it had mulberries growing on it, I would probably call that a definitive identification point. And yes, I too had noticed the strangely lobed juvenile leaves of the mulberry: first time I encountered them, I thought the tree was mutating!

Thanks again to everyone for their suggestions.

Rachel