Sticky Fn key?

When the force is with me and I begin typing in auto-pilot mode (not looking at the keyboard anymore), I sometimes find that the keyboard is behaving as if I am holding down the function key.

I presume I have hit something that makes the fn effect persist. I cannot findout what is causing it and am unsure how to turn it off...by that time I am hitting keys randomly and it goes away.

Any ideas?

Is there a resource where I can find out more about the keyboard other than experimenting with the keyboard viewer?

Mike
PS I am using a British keyboard setup.