Thankyou guys for your advice .
Edward ... Yes, I did read the instructions beforehand , but for all the reading , you only really learn when you try to do it . I know FLR'ing will reduce case life and I only did it because of the problem . Good pictures , thanks . I will try smoothing the slot edges . Your collet die cap looks to be made of steel . Can you confirm this ?
Son ... Yes , I also thought it important to trim the cases (bit of a chore with the little finger & thumb trimmer) but I have since read (somewhere?) that the military .303 chamber is oversized to allow for trench crud etc., so no need to trim !!! I think I'll continue trimming to be safe .
David & MkVII ... No I didn't buckle the case ! I thought I had , but it was the collet cap thread stripping off . To quote part of the instructions - "The neck is sized at the very end of the stroke. It requires considerable force on the press lever."

I'm now thinking (from the above info) it may be caused by rough collet slots combined with an alluminium cap . Or am I still missing something ?