Live Fast, Die Young
SSDs hold their data in flash RAM, like a USB stick but mapped in the old-fashioned way that spinning disks do, in what are called "sectors", a name derived from the cylindrical geometry of old-style disk storage. Being flash i.e. built down to a price and prone to eventual failure at the chip level, the controller in the SSDs can relocate the data in a newly-defective sector to a spare piece of extra memory reserved for the purpose. However this extra space is not infinite and it seems the spare-memory limits on my SSD are close to being exhausted.
I've cloned the SSD drive onto a spare HDD I had to hand and swapped it out. If you need to clone a Windows boot disk and are looking for recommendations I used the freeware (for non-commercial use) AOMei Backupper tool. It works from the desktop, no need to reboot to a live CD or disconnect drives etc. and it will copy your live boot disk to, among others, a SATA drive in an external USB drive housing and make it bootable in the process. Much less faffing around than the previous times I've had to clone a boot disk. I can't guarantee it will work in your case but it was no hassle for me (this time).
I've ordered a new SSD, roughly the same size but a bit faster (SATA-3 rather than SATA-2) and with a three-year warranty. When it gets here I'll reclone my existing HDD onto it and hopefully that will be it although I'll be keeping a close eye on it in case it decides to leave a good-looking corpse behind it too. I'll see if the failing drive can be RMAed for a replacement but I'm not sanguine about it.