Aidan;

I've found the OE (German) ball joints are Superior and can be identified by the metal dustboot retaining spring, as opposed to the replacement (British) awfull ones which had a rubber band(!) to retain the (thinner) dustboot and are not very good (maybe made of Taiwanese S*** metal, or the same metal they used for making pretzel MG blocks). There is no reason other than length of expected service life why you couln't mount mismatched ones, but I wouldn't even consider mounting the junk ones on my car if they were given to me (just my opinion). See if you can get your money back for the junk one...quality always stands the test of time!!!

Cheers