How can I remove the tap handles off my Prestex Bath/Shower mixer taps?

I have a leaking Prestex Bath/Shower mixer tap and knowing that I need to change the washer I tried to remove the tap head. It appears that the tap head is fixed down with an integral, hexangonal shaped indicator and thin screw down into the tap. I have removed this and yet the tap head remains unmovable. There are no other visable screws that could be removed and no bolt heads visable when the tap is fully open.
The taps are about 30 years old, I would say.
Is the tap head just stuck or is there another magic way of removing the tap head.

Graham Young
November 2007