Update: I kept looking at the grain in this image scan and thinking, it doesn’t look right for a T-Max 400 film; I was right.
It was in fact Ilford HP5 developed in Rodinal: the confusion arose because this image was at the end of a series I had made of storm clouds. I changed over to T-Max ( having run out of Ilford) and continued, but kept the negatives together as they were all of the same subject. That will teach me to pay more attention to the edge markings, getting lazy !! with using my Nikon Df/800 and always having the metadata confirm what my settings were.