I think the wording in the announcement just means that its the only notice that will be placed and they wont be 'personally' asking people to attend?
Looking at Grave N690 on that page it has an entry in the column relating to register of purchased graves (no 1468) so 'may' have a headstone with one of the names in the register.
I am not sure how the sections were laid out with grave numbers (these things are often not sequential in some cemeteries), but if you ask the cemeteries dept, they should be able to tell you if space N689 is adjacent?
I really don't think there will be anything visible to tell you the exact location of grave N689.
Good luck with it, let us know how you get on.
Boo