Well I've had fun today trying different variations and it's worked differently every time.... when I tried to reset it earlier, it worked just the same as you described HarryT, it switched off and I had to press the power button to turn it back on, then it went through the full reboot.
I then tried holding it for little longer (20 seconds), this time it did reset on it's own. I've just tried it again now and whereas before when I held it for around 12 seconds it just went to sleep with a blank screen (resumed within seconds), this time it reset again. When I hold it for 7-10 seconds it goes to sleep with a blank screen.
One good thing with all this fiddling around with the on/off button, it had previously felt a bit spongy, now it slides along easily the same as the other Kindle we have
