Thanks, TeeGee, that all sounds very plausible, but I haven't watered my brassicas at all, and it was at the end of a row! But of course, the terrific rainburst we had one evening about 3 weeks ago could have had just that effect, it seems the most likely solution. I cut another today, that had just a tiny beginning of the same rot at the base of the head, easily cut off and the head not affected. The taste of this cabbage (raw, I eat it raw) is out of this world, I just hope the rest of the crop is unaffected and won't go to seed too quickly. Brassicas are on such a knife edge as to being worth the effort - my broccoli has been rubbish this year, the main heads about the size of what I'd expect from second sideshoots!