'Speedstep' wasn't specifically the problem, but it was a setting in the BIOS.
For reference, to access the BIOS, press and hold <ESC> as soon as the machine boots up, (when the Toshia logo is displayed). Then, when prompted, press F1 to access the BIOS.
The setting that I changed was to disable "CPU Sleep Mode". I did this by:
Setting "Battery Save Mode" to "User Setting"
Setting "Processing Speed" to "High"
Setting "CPU Sleep Mode" to "Disabled"
$ cat /proc/cpuinfo | grep "cpu MHz"
cpu MHz : 299.947
I should point out that there may well be a way of switching of CPU Sleep Mode in linux, but I don't really know!
Thanks again 2Gnu!