Tree ID please

Anyone recognise this tree, please?  It's growing in Tampere, in mid Finland, and I'm pretty sure it's not a native - we saw it in a couple of parks.  





Rough bark, characterful shape;  long heart-shaped opposite leaves with crinkly edges...



...and clusters of pale lemon, petal-less flowers buzzing with bees in early June.

Any ideas, anyone?  I'm stumped...

Thanks in advance!