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.
The passage describes Slater's fate after "To Hull and Back," highlighting his criminal activity, his declining relationship with Del and his gang, and the prequel's setting.