Sorry for the paragraphs, formatting this on my phone is too clunky so I will just address your points one by one:
Yes, it worked for 2 days running on the HDD and then just stopped working out of nowhere. I was playing a game at the time which meant it was working at a higher level of performance but it had coped fine with the same level of use over the 2 days prior.
I installed the OS through a usb stick, but you are correct that the only immediate damage was the SSD, followed by the system dying 2 days later.
I haven't got another system to test with, but the monitors are fine and displaying picture, so both are working normally. My motherboard doesn't have any video output so the only way to connect them is via the GPU. If I am receiving no picture, does that mean the GPU sustained damage too? Or would the faulty motherboard just not recognise the GPU and subsequently not attempt to output any video?
The power button works to turn the system on, however then is permanently on and doesn't turn the system off after holding down for any length of time. The only way then to turn the system off is by the PSU switch at the back.
Thank you for the clarification about the suspicion of the faulty motherboard. My main concern now is whether my CPU or GPU may have also sustained damage. I will be buying a new motherboard within the next 24 hours, so are the components safe to transfer on to a new board? I haven't had anything like this happen before so don't want to take unnecessary risks. I really appreciate your help.