Hi, I'm afraid that it's going to be a tank out job to get at the pump/sender unit as, unlike the classic, there is no access from inside the car. It's possible that it might be a loose connection but most likely the sender unit has packed up (not as uncommon fault on P38's).
Mine is playing up too - will only register at all if there is between 1/4 and 3/4 of a tank and only accurate if there is between just under 1/2 and 3/4 full. Otherwise it shows empty.