There have been a number of complaints about the flimsy plug-in couplers Hornby is fitting to locos and rolling stock (specifically the new Pullmans). I have a Black 5, and wrote to Hornby about the couplers. In their usual fashion, they sent me several "new and improved" replacements, but of course they weren't new or improved at all.
While re-wheeling some assorted second-hand goods wagons, I came across a Dapol wagon with couplers that plug into a pocket that has a clip-on plastic cap. I found that these are interchangeable with the Hornbys, and they are much more sturdy and have nice metal hooks. I haven't seen this mentioned here so I thought I'd pass it on in case you have any Dapol wagons lying around. I put the Hornby coupler on the wagon and marshalled it near the end of the train. Of course you could use Kadees.