After lots more Googling, it seems that the laptop battery could be the cause of the problem. I pulled the battery out whilst the laptop was connected to the mains and the CPU usage immediately dropped to 0%. A reboot (without the battery) has restored the mousepad and keyboard to proper use, and everything is back to normal.
I'd installed a program called Process Explorer which showed the CPU usage in much more detail than Ctrl+alt+del - this showed the main usage was for Deferred Procedure Calls (see Wikipedia if interested) which led me on to Googling similar problems that were caused by batteries.
So it looks like my battery is knackered, almost certainly due to the fact that the laptop is always used at a desk on the mains, and is rarely run off the battery. Now that I think about it, the usable life of the battery has deterioated recently to the point that it says you need to switch to AC power almost as soon as it's has been unplugged.
Vindaloo