Scale insects. https://www.gardenfocused.co.uk/fruitarticles/plums/pest-disease.php
Bleurgh. Thanks both. I'll hit them with a toothbrush as that link says -they're a bit weird looking...
You could soon be self sufficient in organic coccineal substitute.
@christianSpaceman started
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Scale insects.
https://www.gardenfocused.co.uk/fruitarticles/plums/pest-disease.php