How to add page content and webform as tab in a page?
There are two ways to do that : 


Write a custom module and create a menu path "node/%node/enquiry" and return your enquiry form programmatically. In hook_menu you can set type = MENU_LOCAL_TASK that will display the path as tab.
You can create a view page that displays webform and select the menu Menu tab under menu setting. Path of page should be node/%/enquiry, so that It will create a tab on node/% page. Screenshot for the second method :