It was about 2cm long and sluglike in appearance, but with a segmented body and a short thin tail. Ordinarily I would have put it in a box to see what it did, but at that hour looking at it made me feel quite ill and so it had to be washed down the plughole after some Calvin & Hobbes-style 'eww! Ewwww! Eeeeww!!'-ing.
Any guesses? A web search for 'slimy household creatures' hasn't turned up anything useful.