I think i know why the circuit isnt working. From my measurements i noticed that if the input voltage was about 7v, the end of the zener was about 1v when it should have turned off the base of the transistor. Looking at the characteristic of the zener diode it shows that its reverse bias voltage is only maintained when a minimum current is flowing through it. So as the input voltage lowered, the end of the zener connected to the base would fall and turn off the transistor, this would gradually stop current through the zener, and its characteristic shows at less and less current its reverse voltage decreases. So this would mean the end of the zener voltage would start to rise and it would turn the transistor on again so that current would again flow, which would make the zener voltage again try to fall to maintain its zener value and try to turn off the transistor again etc etc. I had to take the input voltage down to about 5v to actually turn off the zener!!
This is the only way i can think to explain the measurements. Or am i completely wrong.