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It looks like a pretty solid C. truncata based on the shape of the rear edge of the central band. The projection isnt long enough for citrata but without a view of the underside to make the confirmation.....
It pays to contain these in a transparent plastic box of some sort so that the underside can be snapped also.
You're right, of course. My set of specimen jars arrived a few days ago, so they'll be put to good use in future!
Lat/Lng: 52.3, -1.6
OS grid ref: SP3376