PC now back up and running, but was was completely unable to reinstall either Windows or Linux on the old Motherboard even after clearing CMOS and unplugging all devices apart from main SSD, seems like it may have been a failed BIOS update as was unable to flash to my previous (known good) BIOS version or indeed any other versions.
And this is why PCS don't allow you to flash/update the BIOS whilst staying within the warranty I'm afraid, cos that can unfortunately happen if it all goes horribly wrong
I'm glad you managed to find a replacement mobo and get it sorted though and glad that its all running again