Thanks for response. My initial reason for believing IRST was at fault no longer stands up to scrutiny. The method I used was to install the HP Softpaqs one at a time and check for the delay after each, and it appeared after installing IRST, but now it is there permanently even after a further clean install of Windows. The BIOS in this machine is so simple there are no settings for SATA control whatsoever,,,, So I am no closer to determining the culprit, I thought IRST fitted the bill as it would affect the POST, guess I shall have to look elsewhere...
Saga continues. with the Drive removed normal access to Setup was resumed, so next step was to ensure that there was nothing lurking on a drive that would cause the problem, so I cleaned an SSD with Diskpart and reloaded Windows. Same problem. Out of curiosity I tried Hibernate instead of shutting down, that worked perfectly. Not only that, but the fault disappeared! So I proceeded to load some drivers, all fine until I installed IRST. Fault re-appeared, and was corrected by choosing Hibernate just once. Puzzled...