Last summer I moved into a new house and it has this tree in the back garden. Can someone please help me to identify the species and also to work out why it is not growing very well.
Some branches are out in full leaf with the seeds hanging beneath, whilst other branches (probably just over 50% of the canopy) show reduced growth and poor-quality leaves.
Updated photos 15.05.2019
The photo below shows a damaged leaf which may have originally misled, apologies for that.
Additional 2020 video footage from a drone: https://youtu.be/S-vcsmApJ7g
UPDATE 20.07.2021: The tree was pollarded in Spring 2021 and is now shooting as below. Which branches should I focus on keeping and which should be pruned off to preserve the best properties of the tree?