Metal fatigue caused by the front of the shell flexing, I'd like to bet that the "A' posts have rot well set in around the area on the inner wing side of where the bonnet hinge mounts. The crack is slowly working its way around the screen pillar at its weakest point! Recognising the problem, and correct rectification are crucial at this point! Removing the wings for inspection of this area is about the only way to find out for sure!

Also worth inspecting is the area at the base of "A" post, where it meets the floor and sill area, and ramp lift bobbins on the underside of the floor pan, if this part of the car is rotten, then all stress will go to the cracked area!!
Edit, just re read your OP, seems you've answered your own question!
"However after jacking the car up, the crack has reappeared through the filler and paint which raises structural concerns with me"
Thom