re liability in clearing pavements of ice/snow

I think this is true. It is called misfeasance. It is like negligence except you have actively taken action, but made the situation worse.

Removing a top layer of snow might expose patches of ice underneath.
And gritting this ice will not melt it straight away.
And grit can be slippery in itself.

And even if you have done this all perfectly, can you prove it, if someone slips - perhaps on ice already on their shoes?

So it is safer to leave it. And yet we would be better off if everyone had a go.

Perhaps the answer is to clear the ice/snow anonymously in front of someone else's house! The net benefit is the same - just don't get caught.