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If you can revisit and break open a small twig longitudinally there should be interupted pith like walnut.
I should have known that one straight away, I'd forgotten about their suckering habit, and was thrown by the apparent copse of them.
As you say, compound leaves are similar to Tree of Heaven (Ailanthus altissima) but on this tree, the leaflets are not neatly opposite, but are staggered, and somewhat floppy.
Mark, I didn't know that they had chambered pith, so thank you for that comment. (memo to self, take knife next time am visiting an arboretum.. only joking!)
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 51.591513888889, -0.125675
OS grid ref: TQ299897