I agree with saltcedar - the very invasive "Tree of Heaven" (also called "stinking sumac" but it is not a sumac). After cutting it down, dab some brush killer on the cut stumps, especially around the outer edges. Retreat if it sprouts again.

If the leaf is shiny, I believe it is Morus alba, the non-native white mulberry. That is what I've heard. The native Morus rubra has a dull upper surface.

A good picture of the trunk bark also helps in addition to any picture of the cones. If no cones are in reach of the camera on the tree, perhaps an older cone can be found on the ground.

Do you know about this group? Looks like the involved public agencies - including the one Council - aren't the only interested parties. Maybe you can find out more about the plantings from them. http://www.ashtongardens.org.uk/about.htm >What's their problem?< From the page linked to above (read between the lines): Through public subscription we funded a leading charity law barristers opinion to show that the Gardens were a charitable gift held in trust for the people of the town. The Charities Commission has written to the Council to remind them of the need to register and regard the gardens as a charitable trust. We opposed the planning applications that would damage the cultural and historic integrity of the Gardens Our members worked with the Garden History Society and English Heritage to have the Gardens listed. We successfully lobbied for the refusal of the first Safeway plan to turn part of the gardens into a car park