Thank you for the reply.
Yes I believe I meant the bookmarks that are created as a popup TOC in my reader (and as a side bar in Linux pdf readers).
I have tried your suggestion and it did bring some changes, though I am still a bit bewildered as to how to get just the top level eg:
Chapter 1 -->Headings -->Sub-headings
Chapter 2 -->Headings --> Sub-headings
Chapter 3 --> Headings --> Sub-headings
I only want Chapter 1, Chapter 2, Chapter 3 and none of the other nested headings.
I shall keep experimenting and see if I can happen on to it.