Hi Richard,
I would completely stay away from drop downs. You often get problems with different browsers and mobile. For main navigation you could go for: Home> What we Do> Who we are> Contact. Next to that I would work with a sidemenu or footermenu in which you have all the services. For example you could take a look at www.aliexpress.com. This way the visitor can navigate easily and all the serbices are visible on the homepage for visitors and Google.
Good luck! Tymen