A load of us fixed wing boys at our work did the same thing and it appears it depends on who gets hold of your application with regard to getting stuff back dated. All using the same letter with the personal details changed.
We have had.
code changed refund on headset and previous years refund including the travel 100quid.
code changed reciept required for headset previous years refund including travel
code changed no reciept for headset refund previous years and reciepts required for the 100 quid.
And a whole heap of variations on when it can actually start ie. After training finished, when started work as an instructor, when actually got a line job.
And this was out of the same tax office.
Mind you that office is well renowned for differences between case officers. People have managed to get tax back on training bonds others got laughed at.