Hello, @seanh,
Please correct me if I am wrong, you want the same menu item as book now on your site and when click on it, the new page will open having the form on it, then the user will fill out that form. Right?
If so, then we suggest you to please navigate to WP-Dashboard >> Pages and create the new page, set your form in that page using Custom HTML or using your booking plugin, and then publish it.
After that please navigate to Appearance >> Menu and add that page as a menu item from there, save changes, and check back to your site after clearing the browser cache.
Best Regards,
PThemes Team.