No. I've had a further look at all the bits on WIS and EPC. I now think it must be a fault with the level sensing. But there are two level sensors, one in each half. They are separately connected to the ECU, but share an earth return, also via the ECU. They are claimed to be variable resistor types. I was expecting them to be PWM like a lot of Saabs, but then remembered that it's a Bosch system in the V6, so maybe there's a difference.
A sthe ECU is involved, I may be able to find some info via Tech2. I was wondering why the gauge dropped to zero when there were two level sensors, but maybe it's a failsafe system controlled in the ECU, which also bodes well for a Tech2 check.