Bingo! I think these are the culprits ....

My very lacy-leafed rhubarb is trying hard not to give up ... so you can imagine my delight in exposing more than a baker's dozen of these naughty munchers.

I've done my best snail-flinging exercise today ... way down the back alley. But I've been told that they travel far and fast, with something like a homing instinct! Grrr .... I'm going to keep an eagle eye out for them now.