Thanks to everyone who provided help with this failure of the 4.9.0-5-686-pae kernel. I have tried both "nopti" and "pti=off" kernel parameters to see if it is the fix for "Meltdown" which is causing the problem, but neither parameter changes anything. Since the system doesn't even get to the kernel fsck stage, I don't have any logs to be able to either report a bug or analyse the problem. I also realise now that auto-update is turned on by default in Stretch, so that is how the new kernel became installed. I have temporarily turned auto-update off because I don't want any automatic update to clobber my only working kernel - the 4.9.0-4-686-pae. The updating system only seems to keep 2 generations of kernel?
Any further ideas would be very welcome.