Bang on the money, thank you Mike-Bracknell!
Having searched the ASUS website manually without success and performed endless searches of the internet similarly without success, I went back to ASUS. On this occasion I found a utility that I'd missed before which when installed runs a check of the motherboard and lists any recommended downloads.
Lo and behold, an S3 fix was listed and which has fixed the problem.
I think you are probably right in that this fix came factory or system-builder (MESH) installed but got lost in the software rebuild. It is now safely saved in my backed-up "Software Source" directory (to be precise, in the "install on rebuild" subfolder) should I ever need to rebuild again.
TVM,
XV