Thanks everyone.
It appears that this was such a common, and indeed necessary, procedure that the 'access frames' and grab rail were fitted for the purpose of allowing a passenger (hopefully!) to carry out this work. I still find it hard to believe that a spark plug can be changed on a running engine, having witnessed the result of one unscrewing to the end of its thread and zipping off over a hedge, never to be found again- I was following a BSA 441 Victor at some speed many years ago.
Regards, Weeds