A long time, yes, but management have their own day-to-day jobs to keep on top of. Investigations are rightly thorough and can therefore be lengthy. Once all the info is pulled together, the RAIB/ORR will probably want to see it all and the ORR maybe even question persons concerned further. Factor in other external stakeholders involved, management annual leave, other incidents requiring investigation and probably a lot more than springs to mind and it’s understandable. Yes, still awful for those involved though.
***edit to add, is it Northern or TPE that are still investigating? TPE train but Northern depot?