A last minute penalty convertion from Paul Wotton sealed the points for Argyle although Williamson felt that thay had good claims for two other penalties before that.
"I felt the penalty was long overdue. We had a couple of valid claims and we never got them," the Argyle boss said after the 1-0 win.
"We're pleased. I felt we deserved the points today.
"We worked very, very hard, had most of the play and most of the attempts on goal. It would have been a sore one to take if we'd dropped any points today."