So I managed to reload Windows, update it, install all latest drivers, run SSD check, run GPU & CPU stress test with no issues. I then was rebooting to run memtest86 and had the shutdown happen. I've got it up again and it's been running memtest for the past 30mins with no errors.
It does appear to be either Motherboard or PSU at this stage. Does anyone know any checks/tests I can run to help narrow it down?