The nearest I can find is Paper Barked maple, acer griseum

Thanks Sue. I'm intrigued - how do you search? In fact we have an Acer griseum, and I've just walked up the garden to pull off a leaf to compare: it is a VERY similar shape, but is dark green though they both feel slightly furry. The trunk of the paperbark is dark brown and shiny, and the trunk of this little tree is grey and rough.

However, I think you are close, and that it is a form of Maple!

it reminds me of one of the sweet chestnut but cant think which one.

Sheila one of my tree books has a section on differently shaped leaves and this was the only one I could find that matched apart from the colour. Slightly furry doesn't quite fit though does it? I would think the look of the trunk might well change as the tree matures - its only a baby! Or perhaps its a cross?

SBG the sweet chestnuts I've seen have had much narrower leaves with more serrated edges. Perhaps there's another one.

I think you must be right, Sue. Thank you :)