Norway maple, I'll wager. There is one along a bit from my work window, just coming into flower as yours is. It is always just ahead of the local sycamore, has leaves that are similar, but it holds its flowers erect rather than dangling under leafy umbrellas as sycamore does. Bees often work sycamore in light drizzle and I reckon these umbrellas help keep the flowers and bees dry.
http://www.woodlandtrust.org.uk/lear.../norway-maple/
G.
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