Mission accomplished. Version 2.6.32-30 of the Linux kernel was downloaded and installed by the update process. I subsequently rebooted to find this kernel occupying the default position in the boot menu. I then removed Version 2.6.32-24 of the kernel, using the Synaptic Package Manager, and rebooted once more to find my boot menu consisting of:
Linux 2.6.32-30 Linux 2.6.32-30 (recovery mode) Linux 2.6.32-29 Linux 2.6.32-29 (recovery mode) Vista Linux 2.6.24-24 [Ubuntu 8.04 LTS]
I must have blinked, because I don’t remember running sudo update-grub to update the boot menu. But, perhaps removing the kernel package in Synaptic automatically updates GRUB2’s menu. It should do since, clearly, when a new kernel is downloaded, it is automatically added to the boot menu. Still, I guess I will have to double-check on this next time I remove an old kernel.
But, for now, it looks like I have finally cracked the operation of GRUB2 and its weird and wonderful collection of script files!