Rats seem to find the taste of the fuel pipe rather attractive, I've had mine damaged in this way and have been told that Ducato owners in particular are having the same problem. I've had mine repaired roadside by the AA with self bonding tape. Is there a repair rather than replace the whole tank and pipe, such as plastic welding. It will need to be MOT compliant

I know rats can chew through steel too but my suggestion would be to go for a braided line.

Do you mean the pipe from the filler into the tank? The pipes under the car should be protected by a ‘tray’ clipped over them.