I want to give testflight a try with an Enterprise build, however when an enterprise build is created within XCode a url where the build and manifest file are located msut be supplied. But I can find nothing on the testflight website that says what this url should be.
TIA
Xcode will generate a boilerplate plist file containing the URL of the IPA. The URL would be where your app will be hosted.
Since you're using TestFlight, you don't need this. TestFlight will handle this for you.