Gold medal for observation goes to Mark!! I thought I'd got away with it!
When I bought the drill table up to the barrel trimmer it caught the edge of the cap blank and chipped it. It was a case of all or nothing at that stage because I didn't want to disturb the clean lines (I could have segmented it), so I trimmed about 2mm off the end of the blank and tube to get it clean and square and carried on. It actually came out better than I was expecting, but obviuosly not good enough to get past the experts.
You got me and I promise I'll come clean from the outset if there is a next time!!