Ever seen a snail like this before?

Having been in touch with the Concological Society of Great Britain and Ireland, I have been informed that this is a 'Cornucopia' version of the common garden snail. It is uncommon and can be caused by genetic mutation or damage to the mantle. Mystery solved.

Maybe don't throw that one on to your neighbour's allotment

@Gigit Most of the snails from my plot go over the fence into the graveyard nextdoor.