Thanks, lots of good suggestions! from a brief bit of research i think my top ones are pickle pie, mxr fuzz deluxe, and depth charge.
I love the sound of my setup now, which is a black big muff through a Tym Big Bottom (a great bass loop filter thing), but I just want something simpler and smaller.
The Tym Big Bottom is fantastic - it splits the signal so that the low frequencies go straight through to the amp clean, while the mid/high frequancies go into an effect loop where you can put any pedal. So I get clean low end and fuzzy high end - amazing.