A theory...

Well I caught one slug motoring towards the plants and then diverted by the worryingly blue pellets (I bought them from HDRA - they must be OK!). Here's a theory then - slug/snail climbs leaf to nibble it, leaf can't take weight especially in recent heavy rain and snaps off at weakest point. Works for me. Just going back out to check the slug isn't having a final meal of meconopsis before he crawls away to die...