Further developments...
Intel have offered to swap this SSD out for another one the same. It's good of them, but I don't think the problem is with the specific unit.
I tried booting the PC with the power connector off the SSD, then pushing it on as soon as the PSU fan started (after something like one second). The subsequent boot-up drive search found the SSD with no bovver.
So it seems to be that the SSD has to power up a second (or maybe less) after the rest of the PC. I've thrown that idea at Intel to see if they have any suggestions. Meanwhile I'm building a little relay/resistor/capactor/diode device to provide a brief delay at power-on. It may be a kludge, but if it gets my SSD to accle, I'll live with that.
The next interesting bit will be copying the 100GB "Drive C" off the big 1TB (partitioned) drive to the SSD. Intel's supplied utility seems unable to handle partitioned original drives, but I'm hoping EASEUS and EasyBCD between them will do the job.