Welcome, Andy.
I'm going to have to confess my part in the wrigglings of that slug.... I found it the previous evening... climbing down a wall in the direction of some quality bulb fodder.... it was late, ad rather dark... I grabbed a nearby metal plant label ( one of our own, made from chopped up venetian blinds... nice sharp edges) and gave him a hefty swipe with it... he fell from the wall and I went indoors, thinking he was done for. Next day Ian spots the patterns and, as well as the very obvious trails you see in the pictures... there was also a lighter trail... leading from where my supposedly dead slug fell the night before. It seems I only wounded him and he crawled off and made these rather fab tracings as he went.... perhaps his injuries resulted in the extra patterns.... who knows.... trying hard to feel guilty about maiming him instead of dispatch with a quick death.... but he has gone to the great lettuce patch in the sky now.... for sure.