Could I just throw a couple of points in the pot.
If the seat Safety Case was invalid, or even non-existent, then the Whole Aircraft Safety Case, for a number of user platforms, is compromised. On Hawk, that implicates the Aircraft PT, the Release to Service Authority and Aircraft Design Authority.
I am not entirely convinced a RTI (Routine Technical Instruction) was suitable here. It may only be used if "Design Organisation (i.e. Martin Baker and BAeS) input is NOT required".
Does anyone think MB or BAeS should not be involved in any way whatsoever when there is a perceived need to (a) crack detect and (b) disturb a safety critical system, including replacing a component whose correct fitment is critical to correct operation, and is known to be so?