Ooo Bertie this is very Virginia Creeper looking, the flower buds are very similar but I don't think the leaves are right.. not much help I know, sorry, interested in what it is though..

It does look like a very heavily dissected Parthenocissus quinquefolia Engelmannii..... Could it be a species of Cissus? It's not a genus I know very well, most of them being purely tropical plants....where did the cutting come from Bertie?

Hmmmm, it could be Ampelopsis brevipedunculata (Porcelain Berry) a close relative of both those genera. The narrower leaflets look possible.

Thanks for the suggestions, gives me a start to do some looking at Google images.
I found it growing at a chateau called Valmer near Tours, where somehow a cutting mysteriously found its way into a plastic bag in my pocket... must have fallen in, I guess. There it was growing rampantly and was covered in bees, which is why I thought it would be good to grow.
Looks like the flowers will open soon which will should make identifying it easier.
Looking at the pics on Google, the porcelain berry looks very promising... I shall have to wait and see if it makes purple berries after flowering.

I think the arrangement of the long, narrow leaflets - almost completely separated from one another - is rather different from Ampelopsis brevipedunculata.... there is a more heavily dissected form:

http://my.qoop.com/store/Virens--Latin-for-greening--4781843075268104/Ampelopsis-brevipedunculata---Porcelain-Berry---deeply-dissected-by-Virens-qpps_390495980160495/

but even in that the leaf is very much entire and not divided into leaflets.

Compare with Parthenocissus quinquefolia:

http://my.qoop.com/store/Virens--Latin-for-greening--4781843075268104/Parthenocissus-quinquefolia---Virginia-Creeper-by-Virens-qpps_504640973088244/

Please do keep us updated on this one Bertie!

If I still can't identify it when it flowers, I shall go back to the gardens, find a gardener and ask him/her. Thanks for the suggestions so far.