The AAIB report identifies the issue of the cartridges life as an airworthiness matter and it is however the timex status of the seat had no bearing on the accident.
The emergency services had to deal with a badly injured pilot who was still strapped to a live ejection seat. Presumably expired cartridges are potentially more unstable and dangerous than ones that are in date?
As an aside, given the danger these seats pose in the event of an accident where they haven't been fired, for whatever reason, and indeed the risk of accidental activation on the ground (as demonstrated tragically by the Red Arrows accident) should they even be allowed in vintage aircraft in civilian hands? After all the Red Arrows seat accident happened to very current professionals with ever resource at their disposal.