This is a strange one. If you go to the Argos website and search for GoPro this will not come up in the list.
However, if you type in the actual name 'GoPro Ball Joint', it comes up! Weird!! I'm pretty sure the word 'GoPro' is in 'GoPro Ball Joint'!
Anyway, this is the best price you can get these for anywhere. Doing a 'Completed Search' on eBay shows that they've sold for even more than the pre-offer price over the last few weeks