That fixed up the compile error. Thanks 
But when launching the build i get a crash with an error :
"Fatal error: [File:D:\UnrealEngine-4.8p3_VXGI\Engine\Source\Runtime\Engine\Private\ShaderCompiler\ShaderCompiler.cpp] [Line: 139]
Invalid ShaderCompileWorkerOutputVersion. Rebuild Programs -> ShaderCompileWorker to fix this.
So i guess its down to a shader compiler now. How can i get round this one. Thanks
Really looking forward to testing out VXGI as it will be an excellent lighting solution for my game