Hi Steven Bramall,
Since we use Telerik.Web.UI all skins are embedded into this assemble. In your case you should create a custom skin that will be used by RadMenu control. For more information please take a look at the following url: Tutorial: Creating A Custom Skin
I noticed that you are using Sitefinity where we do not distribute skins files. They can be downloaded form your account at telerik.com. For your convenience I attached forest skin as an zip archive( version Telerik.Web.UI_2009_1_402).
Let us know if there is anything else that we can do for you.
the Telerik team