I know of the fuel/diesel filter under the bonnet on the TD6, but is there a 2nd one under the car or near the fuel tank? Asking as a guy from work seems to think there is?

Nope, one fuel filter only in the engine room.

Most in tank pumps have a gauze on the pickup (generally speaking), perhaps that's what your colleague is on about, but they're quite coarse and are only really to save the pump from sucking up something that might jamb it.