In article , Janet Baraclough.
.. writes

Okay, it's a guess, but based on observation of the recent appearance
of snails in central Scotland (mainland), coinciding with warmer
winters.

Years ago when central Scotland had much colder winters, we never had
snails, but plenty of slugs. Hens eggs accidentally left in the nestbox
overnight, could freeze solid. The frozen contents expand which bursts
the shell. I've always supposed the same happens to snails in really
cold conditions (-17C or so); which would explain why slugs, without a
shell, survive.

... although a snail shell does have room for expansion through the open
end, whereas an egg does not..

But it may be something like - slugs overwinter deeper underground and
are protected to a degree, snails are in crannies in walls. That's a
pure guess.

--
Kay
"Do not insult the crocodile until you have crossed the river"