I couldn't have my gorgeous boy picked-on by the blue brat so I duplicated Cowboy's sounds, aiming them back at him. He swung to look at me with a very puzzled expression - he couldn't believe, I think, that Mum was threatening him! - then he dashed off to the safety of the opposite corner. His trajectory took him within reach of Ratty, who smacked him upside the head on his way past. Just to remind him of who's in charge.
By the time the run was dry, all five were co-habitating quite happily so I left them to it, and only put the ratbag and Tangy back in their own run at dinner time.