Hey, we ended up returning the Motherboard for a replacement. If that doesn't solve the issue does this mean 100% the issue is the CPU?
No. If you just stuck everything back on the new Mobo, and hoped for the best? you could still have a part that is faulty.
We tested the PSU with a paperclip, shorting it to see if it'd turn on and stay on, which it did.
This only shows that the PSU will turn on, it does not mean that that it is delivering the correct current. The PSU must be under load, connected to components, this can be done with a professorial tester. Many computer stores will be happy to test your PSU if you take it into them, maybe ring them first.
Is your computer now working?