Highlight Post Parent Page when Viewing Single Post

hi there

i have managed to get my page structure and navigation working correctly,
using page parent and ancestor class styling for navigation. as to highlight the current page parent etc.

this all works fine for pages,

yet with posts, (blog posts are returned to a Page called ‘DIARY’)
when viewing all posts the ‘Diary’ menu item appears as selected,
then once you click to view a ‘single post’ the Diary menu item appears un-selected like the rest of the items.

Is it possible to have this active page navigation work for posts?
Are there additional css styles i need to add?

I currently have styles for (which work fine for pages):
.current_page_item a
.current-page-ancestor a

many thanks in advance.