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Immature snails can be tricky (and I'm not an expert on them, hence I haven't clicked on the agree button), but I think Cornu aspersum is the most likely option. The similar species are Arianta arbustorum (which doesn't have the dark speckling/mottling) and I suppose the recent colonist Helix lucorum now needs to be considered - as far as I know that is more banded rather than speckled/mottled.
For more on Helix lucorum see the comments here:
and this article from the Conchological Society:
Entomologist and biological recorder
This pattern is typical Cornu asperum - as you say quite distinct from Arianta and Helix lucorum (which apparently is huge when it grows up!)...but not yet seen babies of the latter to reliably comment!
BMIG website: www.bmig.org.uk (centipedes, millipedes & woodlice)
BMIG Newsletters ~ www.bmig.org.uk/view/resource/bmig-newsletter
BMIG Bulletins ~ www.bmig.org.uk/view/resource/bmig-bulletin
Excellent - thanks for the tips Martin and Steve.
Lat/Lng: 53.5983, -2.5178
OS grid ref: SD658114