Hazelnut with irregular teethmarks around the opening.
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I've just read that hazelnuts chewed by woodmice have marks both on the inner rim of the nut and also on the surface around the hole. Vole-chewed hazelnuts have teeth marks across the inner rim of the nut but don't leaves any marks on the surface around the edge of the hole.
(This info came from a handout from the people at www.greatnuthunt.co.uk , which I assume is happening again this autumn!)
Thanks for your comment.
I agree this has probably been opened by a woodmouse as there are markings on the outer edge of the hole.
I have been collecting the nuts as part of the greatnuthunt. Unfortunately though I don't think I have found any evidence of dormice!
Lat/Lng: 51.4, -1.1
OS grid ref: SU6474