Making unrelated page highlight a navigation item

Hi,

I currently have a navigation similar to:
home | advice | communities | services

the current page highlights when in a section (including when in a child page).

How can I make a separate page (e.g. /map) make the communities navigation show as the current section (e.g add a class to that menu item)? – so it looks like you are in the /communities section.

Hope that makes sense.

Thanks for any help.

[EDIT] I should also add that the navigation is generated using the WP Menus.