Not much to go by. Some kind of horsechestnut?

Looks vaguely Aesculus-like, but the downturned flower stalk is wrong for an Aesculus.

The leaves are maple-tree like, I'm not certain if it isn't some sort of maple, but I thought I'd throw it out there and get some other ideas.

Can you show a photo of the whole thing? It is difficult to see how the leaves are placed on the stem.

Acer pseudoplatanus? It's hard to see but it looks like the opposite leaves are offset 90 degrees on successive nodes and the young leaves/flowers look like that species. Here's A. pseudoplatanus 'Prinz Handjery Dawes':{{gwi:354602}}

This post was edited by formandfoliage on Tue, May 6, 14 at 11:00

Yeah it does look like that.

I'll get some more pics up later on.

Regards
Nick

Yes, definitely a sycamore (sycamore maple over here).

It's Acer pseudoplatanus aka Sycamore in British English. In a large space it makes a handsome tree but is essentially a weed in our typically small gardens due to its propensity to self sow with abandon. An Acer which just 'shows up' here is almost always a Sycamore.