Oh ****, not again. I noticed the CPU temp hitting 40C (needless to say, noisier fan) when I'm hardly doing anything and CPU usage has been at zero for some time, and sure enough I found a thin layer of dust over part of the heatsink. I decided I've finally had enough of this stupidity (as the last time I vacuum-cleaned it out was when I started this thread in August), found a thin piece of packaging sponge, tested it for air throughput (vacuum cleaner), and taped it to the side of my case to cover the vents. CPU usage is now at 32C (dropped steadily from 36C after boot). I'm happy to secure the sponge a bit more thoroughly once I've run the system for a bit longer.
One interesting thing is that every time I've checked the graphics card (which is right at the bottom of the system), it has never needed vacuum cleaning.