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I think this is the"large" variant (S. putris) - the size name is quite misleading! The "small" (S. oblonga) is 6-8mm with larger first whorls. Large (S. putris) usually 10-17mm with close, small first whorls.
Thank you, em663.
I hadn't realised there were two species until faced with the names here and then foolishly thought that general size was indicated!
I will attempt to change the id.
Lat/Lng: 52.5204, 0.2496
OS grid ref: TL527937