About 5-8mm shell diameter. The shell is wider than it is tall
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I would say Kieran, even looking at the original image, that this is not Discus rotundus. There should be exquisite ribbing across all the whorls.
I cannot say, for certain, what it is, sorry.
I have just posted here
THREE MARINE PROJECTS
Lat/Lng: 51.0, -0.4
OS grid ref: TQ1431