I haven't posted anything here for some time, so I think an update is in order:
I had some problems back in October which turned out to be due to the cheapo SD card I was using: On a couple of occasions I found the system working but reporting errors when it tried to write a file. I ssh'd in to find the entire file system was read-only - presumably as a result of some sort of error, but I couldn't find out what the error was because, of course, it couldn't write to syslog. A reboot restored normal service but a few days later the problem occurred again. The third time the system refused to reboot. I put the SD card into my main PC and it wasn't detected. So, having already bought a new card after the first failure, I quickly installed the latest raspbian build on it, installed polly, copied all the config files from the most recent backup (Fortunately I've got daily backups running.) and normal service was resumed.
Since then the system has been much more reliable, and it's currently showing 58 days of uptime. (The all-time record uptime was 550 days, ending in December 2001 when I accidentally unplugged the server! This was the DOS-based version.)