Hi EA_Rtas ,
I never managed to resolve the issue and ended up upgrading my system.
New motherboard, CPU (Ryzen 7 9850X3D) and RAM. The game now runs, so have been enjoying that.
I did manage to reproduce the crash even with the new setup. The only thing that had changed was, I had played another game that required compiling shaders between Zero Company play sessions; exactly the same symptoms, could get to main menu and whole system would die, requiring a hard reboot.
Clearing the shader cache in the AMD Adrenaline Software and recompiling shaders on booting up Zero Company seems to resolve it. But I am very hesitant to play any other game whilst I am still going through ZC!