Validation and verification (V&V) benchmark exercises form an important part of the development and release of the FISPACT-II inventory code and its associated input nuclear data libraries. This paper describes the latest V&V based on the fusion decay heat measurements performed at the Japanese FNS facility. New and novel assessment techniques, focussed on the complex breakdown of decay-heat contributions from individual radionuclides, have been employed to interpret the simulated results, benchmark the data against the experimental measurements, and to compare results from different international nuclear data libraries. Example results are presented and discussed for the TENDL-2017, JEFF-3.3, ENDF/B-VIII.0, and legacy EAF2010 libraries, highlighting both good and poor predictive performance.