They have two basic types of accident inspectors IIRC; engineering and operational.
The operations inspectors are generally required to have or have had an ATPL, often retired senior airline captains. The engineering inspectors usually come from an aircraft engineering background.
I applied a few years ago when there were over 500 applications for 3 vacancies, so getting shortlisted seemed quite an achievement.
Starting salaries were around £50-60K then.