Sorry for the lack of updates on this!
After some back and forth with ASUS and the company I bought my motherboard from, it eventually turned out that ASUS would not repair the motherboard. I had a hunt around for some used Z170 chipset motherboards, but since I need an ITX sized board there was very little around for a reasonable price.
As a result I decided to bite the bullet and buy a new motherboard. Unfortunately that means I had to purchase a new CPU, but I've now rebuilt the system with an AMD Ryzen 7 3700X CPU and ASUS B450-I motherboard. So far so good, everything has been nice and stable. I am slightly concerned at the temperatures as this CPU does run much hotter than my old one so will eventually need to switch everything into a new case with more airflow and better cooling, but for now I'm just glad to have a working computer again.
Thanks for all your help with this issue