Also, casting suggestions:
David O'Hara - Vimes
After I saw him in the Tudors as the Earl of Surrey, I think he's a winner. Perhaps he's a bit too young... either him or possibly... maybe... Philip Glenister. I think.
Samuel Roukin - Carrot
He's tall (check out his photos at the premier for Bright Star, he's freakin' massive!), ginger (okay he has a silly floppy fringe at the moment, but we can fix that) and broad shouldered. His IMDB photo doesn't do him any favours, but I saw him in Happy Go Lucky and I think he'd be as close to perfect as is humanly possible. Also, he looks a bit like the Paul Kidby version of Carrot... perhaps he ought to go to the gym a bit more just in case.
Annabel Scholey - Sally/Angua
She could be either, but more likley Sally. She was Lauren in Being Human (the vampire that Mitchell turns in series 1).