The husband noticed something odd going on outside last night as he put the bins out:
While this pair were hanging on a wall, they apparently often hang in mid-air from tree branches. They're hermaphrodites, and after mating, they've both headed off to lay about 200 eggs each. Sometimes their penises get so tangled up they can't separate, but there's a solution to that - they chew them off. It doesn't put a dampener on their sex life though - they just use their female apparatus instead!
If you're interested in more information about these slugs, just Google 'leopard slug'.