SBDS stands for "Sudden Brassica Death Syndrome" (well, I say it does...). This is what has happened:
I have no idea what caused this. The plant just collapsed, more or less overnight. I suspect that some horrible creature has gnawed through its roots.
Last Summer, when the plants were young and newly planted-out, most of them were attacked by the Cabbage Root Fly and several barely survived. These ones have never grown to the normal size, despite the relatively mild Winter. This is one of them:
This next photo is of one of the spares, which I planted-out very much later than I would have liked, in the big tub which had housed my "Maskotka" cherry tomatoes during the Summer. It looks very healthy, just not very big:
Only two of my six plants (the ones unaffected by the Cabbage Root Fly) have grown to the normal size for this vegetable (which is about four feet tall). This is the best plant of the six in my raised bed:
The other big one decided to produce its first shoots in November, about 3 months earlier than expected. However, it is still alive and is still producing a few spears every now and then.