This is a good question. Only by reading carefully will you be able to pick up on the name Sarum in To Kill A Mockingbird. Sarum is found only six times in the whole novel. Sarum is a group of people.
In chapter one, Scout states that old Sarum refers to a tribe of people, who reside in the northern part of the county. She also says that the Cunninghams are from old Sarum. Moreover, Arthur Radley became friends with them and got into some trouble as they formed something like a gang. Here is what Scout says:
According to neighborhood legend, when the younger Radley boy was in his teens he became acquainted with some of the Cunninghams from Old Sarum, an enormous and confusing tribe domiciled in the northern part of the county, and they formed the nearest thing to a gang ever seen in Maycomb.
In chapter 15, Link Deas intimates that the people of Old Sarum are racist. They are also unpredictable and can be violent. He asks if they can get a change of venue. They are not able to do so. Later on they are the ones who come to take care of "business" but Atticus and Scout stop them.
In short, the people of old Sarum represent odd, backward thinking people.
Old Sarum refers to a rather rugged rural area in the northern part of Maycomb County where "an enormous and confusing tribe domiciled."
In Chapter 1, Scout recounts the history of Macomb County and her family. Interestingly, Old Sarum is the name of one of the oldest settlements of Salisbury, England; in fact, William the Conqueror (Norman Conquest of 1066) established a fort there. While many places in Alabama have the names of locations in the British Isles [Alabama History: An Annotated Bibliography lists seventy-one], the connotation of this name as a bellicose area can certainly not be missed. Most likely, the Old Sarum bunch are probably not far removed from their ancestral clan. (The vast majority of Southerners, especially in the 1930's, before industry came to the South, were of Scot-Irish or English descent.)
At any rate, the Old Sarum "tribe" is a rather unruly group of "country folk" [sic]; Scout describes them as "the nearest thing to a gang" in Maycomb. She adds that some of the male members came to town and loafed around the barbershop, rode the bus to the town where there was a "picture show," attended dances at the county's "riverside gambling hell," and brewed moonshine whiskey. Scout adds that when Boo Radley as a teenager went around with some of the members of this clan,
[N]obody in Maycomb had nerve enough to tell Mr. Radley that his boy was in with the wrong crowd.
As a result of his association with this group of wild young men, Arthur "Boo" Radley was arrested with them for disorderly conduct, disturbing the peace, assault and battery, and other charges. Thus began his cloister inside the Radley house under the supervision of his rigid father, about whom Miss Maudie remarks,
Sometimes the Bible in the hand of one man is worse than a whiskey bottle in the hand of another.
Prior to Tom Robinson's trial, the sheriff and some of the businessmen come to Atticus, asking if he can get a change of venue, as they are concerned about the Old Sarum bunch. Their fears are warranted, as these men later show up at the jailhouse as the stereotypical "liquored-up lynch mob."
After Tom's trial is over, Atticus perceives a glimmer of change in Maycomb County. The attitude of one of the Sarum bunch, Mr. Cunningham, changed enough for him to vote "Not guilty" initially, which kept the jury deliberating for a while.