As far as I can tell, Lady R, the question of the law in Discworld is perhaps best summarized by Terry and Stephen Briggs in The Discworld Companion, where as to Criminal Justice (and Civil)--the explanation is that there really aren't any laws (Carrot threw the only mentioned published copy of the laws of AM at Wonse, and they were picked up by the Librarian and returned (with The Summoning of Dragons) to the safety of the depths of the Unseen University Library. Unfortunately, Terry doesn't seem all that interested in the subject, though I agree that it could be used as subject of satire.
We know there is a Guild of Lawyers who may be hired to represent one in apparently "Guild Courts" (i.e. Thieves Guild), but according to the note, all criminals taken by the Watch may opt to be tried in front of the Patrician. (see p. 248 for the comments on justice for the rich and poor). Apparently one can hire lawyers to represent one in front of the Patrician, since Vimes tells Carcer he'll find a lawyer for him, if any can manage to plead his case with a straight face.
It is generally, however, in Civil Court cases that the lawyers make their money. Apparently there are two court (Magisters for poor people) and a judge appointed by the Patrician for the wealthy.
The only other actual lawyer(s) we meet besides those mentioned by DJ, is the Toad (formerly a lawyer) in Wee Free Men who brought a breach of contract case for his client against her fairy godmother--with devastating results. The Queen, in that book, calls up lawyers (or a dream of lawyers) who are used to terrify the Feegle, but who are outwitted by the Toad.
I agree that it's rather a shame that the only lawyer we see much of is Mr. Slant, the zombie who seems to act for all the groups trying to get rid of the Patrician and or course for quarrels in The Truth between the Printers & Engravers Guild and de Worde's paper. It seems unfair (she said as a retired lawyer) that there isn't a lawyer who crusades for Justice--but since there is no law, justice is pretty much as you find it.