How to display posts from one parent category only on 'results' page
 

I have a site called shackn.com, and I'm trying to work out how to display posts from only one category in the search results page, after a user selects their desired location from the dropdown on the homepage.

Does anyone have any ideas how I would limit the search results to just one Parent category?

Just wondering if anyone has any insights regarding my query.... :)

I'm still keen to get this sorted.

If anyone has any guidance I'd greatly appreciate it - even if I have to pay for the privilege :)