Alright so my GPUs require two 8-pin and one 6-pin PCI-e cables.
First tried connecting the GPUs with the original cables that came with the PSU. It arrived with 4x600mm and 2x800mm 6+2 PCI-e cables. Length should be irrelevant, but I've connected two 600mm and one 800mm cables to each card. Would it make a difference if I replaced the 6+2 with a separate 6-pin PCI-e cable? I don't see why it would since they're designed to work both ways.
Next I tried a custom kit from another brand. This kit only came with 4 x 6+2 Pin PCI-e cables (600mm) and 2 x Dual 6+2 Pin PCI-e cables (750mm). Meaning I had to use a dual cable for the final 6-pin on both cards. This isn't optimal, but should it pose a problem?
I'll try moving the cables to opposite ends first thing tomorrow, haven't tried that yet.
I guess the easiest way to troubleshoot if the problem is indeed the PSU would be to test my system with another PSU. I'll find someone to borrow one from next week if I still can't get it to boot.
Appreciate the replies. Thank you for your time.