Greetings, Everyone.
Thanks for all the advice and shared experience.
It's time for an update.
Firstly, the failed HDD. Actually, it had not failed* and the problem has now been shown to be with the SATA cable! One new cable purchased for the princely sum of £1.03; fitting it has cured the problem. Fit the old cable to one of the other HDDs and it fails to be detected by the BIOS, indicating that this almost certainly was the cause.
Now I had my O/S back at almost no cost, before investing in the Crucial SSD and prompted by SD's comment (which indicated there was probably a gain to be had) I turned to tuning it despite the fact that I actively housekeep the computer and try to do so in a preventative-maintenance manner.
My research lead me to MyDefrag which I tried out after first removing Diskeeper Pro. What a fantastic utility; I can well believe the spiel about optimizing by moving critical files to the edge of the disk to reduce seek time and balancing defragging vs maintaining contiguous blocks of data because the result was stunning. On top of an already freshly "Diskeepered and CCleanered PC with truck loads of empty space on each HDD" I obtained a repeatable reduction in time to XP logon of over 40%. To pick up on your interesting observation, Terraplane Blues, the XP progress bar now completes after 3.75 cycles instead of 7.75!
I then used Winpatrol to check whether I could disable any more services than I already had (I couldn't) as well as to set half a dozen non-critical services to run at stages between 1 and 3 minutes after the desktop has built. The result is that I can now use the desktop with normal performance (confirmed by Task Manage CPU indicator dropping to idle) within about 10 seconds of it displaying.
The entire boot sequence from power-on to usable machine is now about 70 seconds. That's more like it!
Secondly, the new HDD coincidentally required for extra storage. My preferred HDDs have long been Seagate Barracuda 7200RPM on the basis of sustained performance for the usage I put them to and total reliability - now with "total" intact again.
I was staggered to find that the 1TB 32MB cache Barracuda that I wanted today costs the paltry sum of £44.98 from Amazon, delivery included. Now fitted and in commission but how anyone has made a profit on the road from Thailand to my front door in the UK amazes me.
*Just realised that the apparent failure cropped up since I started this post, hence no trace of it previously.