Is there a particular cause for growth like this?
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Terry's identification still stands - my eyes are not good enough to distinguish padus from avium from these photos. My comment is on the flattening/distortion.
This sort of distortion is known as Fasciation, a genetic disruption that can have a host of causes. It is quite common on domestic plants, particularly Roses and Forsythia but I've not seen it on Prunus before. Last year it was quite common on Bryony. The name means "Banded" or "Striped". Someone once told me that the Romans used to carry a rod made of small sticks laid flat so it could be used to slap or slice... can't find any further reference to this, though.
Lat/Lng: 51.7357, -3.755
OS grid ref: SN789055