I got to the end of this (with a few unparsed until 225 puts me right) after a great deal of slog. I am still strongly attracted to the foul-up theory here. Many Guardian crosswords have themes. It's quite common to see "hidden" themes as per crosswords set by Qaos and sometimes other setters, where several answers are are of the same kind but no knowledge of this is required to complete the puzzle. Explicit themes also crop up sometimes, especially on Bank Holidays, with a special instruction saying something along the lines of (to quote the revered Araucaria) "Several solutions are of a kind and not further defined."
However in 50+ years of Guardian Crosswords I have never come across a themed puzzle of the "not further defined" persuasion WITHOUT a special instruction to draw the solver's attention to this fact, so I think that Boatman must have originally intended to provide this and it got somehow overlooked. QED !!