Ok, thanks for your reply, Peggy. I tried matching a leaf from this tree with images on Google and closest similar was a dogwood leaf. That's a good idea to ask on a Uni or Agriculture website, and I've just remembered our local council has or had an arborist on their staff so will try them first.

TimC, you might try this to narrow down your possibilities:

naturalresources.extension.iastate.edu/forestry/iowa_trees/key/key.html

Thanks Tim, quite a useful and helpful site, but sadly, despite the many combinations of searches, I failed to find a match.