Update to my post above
After doing a bit of forum surfing, I got the impression that it wasn't the usb peripherals that were causing the no-boot problems, but rather the power supply. I also went looking on Amazon for a few extra usb chargers and noticed that a lot of the reviews complained of the poor quality of the power supplies and the usb cables on offer (I always read the 1-star reviews when buying on Amazon).
So I changed out the power supply and Nokia phone cable that I was using for a Motorola smartphone power supply and a Kindle usb cable and, hey presto, the Pi now boots with both the keyboard and optical mouse attached. I have soft rebooted several times and had one kernel panic, which was solved by a hard reboot (unplugging the power supply).
Other posts on the forum all point to the Pi being very sensitive to the quality of it's power supply and its associated micro usb cable, so if you're getting kernel panics, try a different (preferably branded) power source and cable.
BTW, my Pi is not yet connected to a network and I'm using the composite video output.