Cochlicopa species
There are two species which might be found in British gardens, and which are very similar. Read the descriptions carefully before deciding.
Cochlicopa cf. lubrica (Müller 1774)
Description: 5 - 7.5 x 2.4 - 2.9 mm. Shell elongate-oval tending towards conical, with 5½ gently convex whorls and a rather blunt apex. There is no umbilicus. Outer lip bluntly rounded, slightly strengthened internally by a pale or reddish rib. Shell translucent, pale to dark brown (sometimes white), very shiny.
Habitat: Ubiquitous: abundant in moderately damp places of all kinds; marshes, grasslands, woods.
Cochlicopa cf. lubricella (Porro 1838)
Description: 4.5 - 6.8 x 2.1 - 2.5 mm. Shell similar to C. lubrica, but smaller and relatively more slender and more cylindrical in shape; whorls less swollen, with a correspondingly shallower suture; apex appearing blunter; shell usually less shiny and translucent, and paler in colour.
Habitat: Ubiquitous: characteristically in drier places than C. lubrica (limestone grassland, calcareous sand dunes, screes) but the two species are often associated.
The taxonomy of Cochlicopa is difficult. Dr. Roy Anderson in "An annotated list of the non-marine mollusca of Britain & Ireland" (published in the Journal of Conchology Volume 38, pages 607-635 in 2005, and revised online here 2008) includes the arguments for leaving the species conditional.
As a "rule of thumb", if breadth divided by height is less than 0.43, then it is probably C. cf. lubricella.
If it is greater than 0.43, then it is probably C. cf. lubrica.