He was sent to prison for diamond smuggling after the events of To Hull and Back. Slater also appears in the prequel Rock & Chips, where he is barely tolerated by Del and the others as a member of their gang, as his friendship with them is at this point obviously falling apart.
Following the events of "To Hull and Back," Slater was sent to prison for smuggling diamonds. In the prequel "Rock & Chips," he's barely accepted by Del and his crew, as their relationship is falling apart.