Well I talked to GKN after they have looked at the photos & they dont think the joint was overloaded, but that it had over plunged causing a ball to jam and break the cage resulting in the damage in the photo above. They of course want the old CV back to see if they can find anything else. One contruibuting factor could have been that the flange to flange gap for the driveshaft on that side increased by 5mm - I did make up a spacer but didnt use it as there seemed to be plenty of spare plunge to cope with the extra length. The current plan is to fit a new joint with the spacer and see what happens. If it or the other one breaks again then we need to look at a different solution.....

New joint to be fitted tomorrow & for my own peace of mind I have ordered a spare one as well!