Hi Paul Diston,
This is a known issue in Xamarin and you can find more details in the below link.
To fix this issue, move both the android sdk and ndk to the location with no space in its path and update the following settings.
Visual Studio: Tools -> Options -> Xamarin -> Android Settings -> Android SDK Location
Xamarin Studio: Tools -> Options -> SDK Location -> Android SDK
Please check and let us know your if you need any clarifications.
Thanks,
Yuvaraj