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... on the bark are found on White Poplar, Populus alba, and on P. tremula (Aspen) and on all the hybrids between the two, known as Grey Poplar.
So your diamond marks are not necessarily an indication of Grey Poplar.
The leaves make it a lot easier to diffentiate, so if you have photos of the leaf litter below the tree,that might help.
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 51.4985, -2.6703
OS grid ref: ST535779