mk 3 fiesta PATS light not going off?

I've got a mk 3 (M reg) fiesta, and for some reason the PATS light on the clock has decided to come on and stay on, even when driving. Now i know normally it will flash when i leave the car and turn off when i start the car adn drive somewhere, but for some reason its decided to stay on continuously whether I am driving or not. I tried cleaning the circuit on the clock/pats board as there was some rubbish on it, but it still remains on no matter what. any ideas would be much appreciated