Search short code category question

I’m using the [gd_advanced_search] short code on four different pages (each page representing a parent category), example here – http://www.preview.pembrokeshirewebdesign.com/eat-drink-stay/

In the category drop down I’d like to just display a particular parent category and its subcategories rather than all categories. Is this possible, a parameter of some sort maybe? I tried category=”8″ but that didn’t work.

Or is there another way to achieve this?

Thanks in advance.