Well folks, I have this evening given my PC a good spray out with "aero duster 101" and swapped the PSU out.
It's now running even cooler, and the Asus Probe graphs are showing straight lines at +12.42v, +4.84v, +3.38v and +1.66v rather than the scatter diagrams that it was previously reporting.
So fingers crossed that the dreaded "two tone horn" may be gone too! Certainly got a clean boot from cold this evening, but only time will tell...
RC