Well, it seems applying seaweed feed did the trick. Later tomatoes on Purple Ukraine, Greek and a couple of others are now unaffected.
I have concluded that the Dalefoot compost is fine without feed for smaller cherry tomatoes, but does not last so well with big beefsteaks. I have to admit though, that one of the beefsteaks, Mr. Novak, was completely unaffected, but he is getting a feed too from now on.