I think this is one the Physidae, but I'm far from certain. The Danded Demoiselle gives an idea of the size.
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Should point out that the reason I don't think it is Large Amber Snail is that it looks too rough on the shell. Not actually the best reason to dismiss it though.
It is definitely either an Oxyloma species or Succinea putris
While I like snails, I don't know much about identifying them. Yet another subject to add to the list of stuff to learn... I don't think life's long enough, to be frank!
I'm in North Hertfordshire, which I suppose is the western boundary of East Anglia. Hitchin is the nearest town - I'm on its north-eastern edge...
Lat/Lng: 52.2477, -0.24862
OS grid ref: TL196625