For anyone who has this problem in the future, I do finally have the S/PDIF
input working with three external digital sources.
The solution was to get a replacement S/PDIF back plate module from Asus, my
original module supplied with the motherboard is labelled SPDIF 1.00-A01,
this new one is labelled SPDIF-SA 1.00. I imagine the SA stands for level
Shift and Amplification which is included on this bracket but not on the
original, the only other obvious difference is the replacement has a three
pin jumper block whereas the original did not.