A novel full-system test (FST) phantom and method have been developed to demonstrate and quality assure the geometric accuracy of image co-registration and overall shot delivery in the context of SRS using Gamma Knife® Icon™. The method uses Vernier scale bars to achieve sub-voxel precision co-registration measurements and pin-located radiochromic films to determine overall shot delivery precision. Validation tests demonstrated that artificially applied registration errors of < 0.15 mm could be accurately detected and quantified. Cross-validation of full-system test results with the manufacturer standard focal precision test demonstrated that both approaches measure similar focal precision errors, to within < 0.1 mm, and that registration and focal precision components of the full-system geometric error can be successfully decoupled using our Vernier FST approach. CBCT co-registration errors were shown to be of comparable magnitude to the focal precision errors, demonstrating that CBCT registration based in-mask treatments can achieve sub-voxel geometric accuracy, rivalling traditional frame-based immobilisation. This full-system geometric test method and phantom design concept is in principle applicable to any SRS technique involving image co-registration.