Defender 110, hence where I'm posting.
1991 200tdi.
Metal.
Because that's the area where the fuel is concentrated after I've wiped it clean and driven round a while. That's why I wanted the exploded drawings from the parts catalogues that were so kindly posted at the beginning of this thread, unfortunately these particular pages seem to have been missed off. I don't know if it's possible for movement to cause wear from the bolt to the tank, whether the rubber washers have a sealing role or are just to absorb shocks when driving. The leak is so limited it implies that it's a washer or seal letting by, rather than an actual hole in the metal. May of course be a split in the seam in the area of the bolts, but I wanted some idea of what it should look like before I started disassembling things. There is no leak on the top or sides of the tank, only at the front towards the axle. There is no fluid leak from the axle or anywhere else that I can see or touch, even after having removed a piece of the floor to see the tank top.