What's this on my tree leaves? x

Please could someone tell me what this white stuff is on the underside of my leaves on my cherry tree x
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Please could someone tell me what this white stuff is on the underside of my leaves on my cherry tree x
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Looks like one of the many types of scale insect https://www.rhs.org.uk/advice/profile?PID=224
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.