Deffo not an FBH fuel feed on my 54 plate, project Drive saw to that
And as it is under the rear passenger door on the OS, I cannot think why it would be water related.
As I noted, the actual Rover workshop manual shows some pipes in this area, but it is so rubbish, it is not possible to really make out exactly what is there.
I had two sets of glasses on, but could not make out from the tiny illustrations whether the pipes vaguely illustrated were for diesel/diesel cooler/charcoal cannister.
It is supposed to rain heavily tomorrow, so this may take a while to sort out.
Best wishes, Frank B