The Sherman is a unit that has been suggested to me by someone else, too. I'm not fully clear on how it makes distortion though - isn't it basically just a bunch of filters and envelopes?
Another unit that has been suggested is Thermionic Culture's "Vulture":
It's a bit more than £800 but if it's really worth it I'd be willing to pay the extra. Anyone have any experience with this unit? :>
-Annikk