Looks like Tilia or Linden to me. Try googling that and see what you think. :)

That's interesting as I have seen Lindens before and it never occurred to me it might be one although the fruits are right. The limes I have seen I recall as having more papery leaves, pronounced narrow tips and are cordate and oblique at the base.

I'm certainly not sure Rhys as I've never had a Linden tree myself. Like you, I thought the leaves had a similar waxy look to a Populus. Someone will know.....

Maybe a form of Callery Pear (Pyrus calleryana)?

Thanks for that. The Callery Pear definitely looks like the right one (I have added a photo from the web which looks just like it). Great to learn what it is as I will be able to spot that in future as the fruits are very different from poplar.