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Going from the shape a perfect match & the other 2 Lymnaea in my book much smaller. I've always seen them as black, but maybe this due to the limestone.
It might well be that Chris, I'd say this one has been dead a while, possibly was a bit darker while alive. The zebra mussels on the shore, ie the dead ones tend to be a chalky white also. Probably paler in the very clear alkaline water as you say.
Lat/Lng: 53.7097, -9.2277
OS grid ref: S@234479