For the record, a rep of Asus Australia has contacted me by PM and make a very generous offer to replace CPU, mobo and PSU and take the old parts off my hands to try and work out what went wrong. As the retailer now has the old parts it's uncertain whether this will be possible, but this is excellent customer service which should be publicly recognised and I'm sure as hell going to try.

Really good job folks, well done. You've just earned a repeat customer.