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The leaves of Alder Buckthorn (Frangula alnus) have edges that are entire the leaves of the plant in the observation are serrate.
Was this growing wild or in a garden?
... (I've seen quite a few recently that have gone wrong somehow) then it's not growing in a garden, but across the lane from Brereton Heath CP.
But you do get odd things planted in hedgerows and roadside verges, every outside of towns (such as the Viburnum plicatum downstream of Astbury Meadow that you thought was probably planted).
I do intend to have another look at The Quinta and Swettenham Meadows sometime (I've missed the spring flowers), so if I remember I can have a look while I'm in the area.
For what it's worth Frangula alnus is present at Brereton Heath.
At a first glance, I thought it was Bird Cherry due to the "skinny" twigs and congested structure.
But I normally encounter that as a tree, not as shrubby growth, so I discounted it.
How absolutely wonderful, Graeme, that you were able to go there yourself and do the bark test!
Thank you for doing so.
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 53.1807, -2.3074
OS grid ref: SJ795648