I love this tree at RHS Harlow Carr which has bright green leaves and lovely flowers. I have no idea what it is though.

Catalpa tree

there's southern and northern Catalpa. and they look very similar. here's what i found about the differences.
i can never distinguish between 2, except when in the southern US you'll see southern catalpa most likely.
http://www.carolinanature.com/...
here's some posts of Catalpa pics from UK - the northern hardy type. It looks similar to yours.
http://wildflowerfinder.org.uk...
i love the purple spotting on yours, i think less spotting with almost white flower is more common.

the 2nd site does not load, i'll fiddle to see why...
it's a list.
you can get it to load by cut and pasting into URL the following below, and then selecting the 2nd item from top
http://wildflowerfinder.org.uk...
in our database here they are: southern
Southern Catalpa (Catalpa bignonioides)
northern
Northern Catalpa (Catalpa speciosa)

According to a couple of web sites the catalpa at Harlow Carr is C. bignonioides.

Thanks! I'll see if RHS Harlow Carr can give me the species name.