Possible Clues:

• Gregarious prisoners?
• Labour Party unlikely to split?
• Working hard, they're linked for some time
• As convicts, its members are bound to work
• Its members have strong links
• Working group of well-connected criminals
• Group of convicts wearing leg-irons
• Linked prisoners
• Feature carrying a restriction binding unspecified number (of labourers)
• Criminals bound to be working