I would think that Duraplug is a reputable company, and to give benefit of doubt, maybe this is a rare failure of quality.
If you can be bothered to post the failed plug (maybe with a few centimetres of cord still attached) to Duraplug HQ, you might strike lucky and get an apology letter and a load of freebies as compensation. EDIT- or maybe just email these photos.... ?
Only guessing though.....
If it was me, I would be far too lazy to go to the trouble, but maybe you are made of better stuff

Feel free to ignore this thought if you want to.