Diesel leak from tank

[Al Housman]

Doka syncro LHD. I've noticed a small leak which is dripping down on the left hand side so opposite the fuel filler. 
Is it possible that a seepage from the seal on the fuel filler pipe could travel that far? I suspect not but have to consider it.
Has anybody had a similar experience and what was the cause please?   I'm pretty sure the tank is plastic and I notice a plate on the left hand side with various pipes. 
If anybody has a diagram of what's what in that area that would be great.
Many thanks

[Aidan]

Al
is it fuel or is it transmission fluid being blown onto the top of the tank by the breathers and then running down, though to be fair that usually shows on the right side.
Left side is the fuel sender which clamps through the side of the tank, two spreader plates in half moon shape with M6 studs, tank wall, then a rubber gasket, and then the fuel sender, then the nuts

the hose connected to the sender is your return from the fuel pump, that would be first thing to check as hose may be perished

this is the pic from Bentley, it's petrol, there isn't a published diesel variant pic that I know of, possibly in the JX specific VW workshop manual  ?

[Al Housman]

Brilliant!
Thanks Aidan. That at least gives me a starting point. The diagram certainly tallies with what I see on the vehicle.
I may be imagining it but I get a sense that it leaks mainly when the rear of the truck is raised up. There is a tiny drip when I have the rear wheels on ramps but when I drove it up to the woods to sort my tractor out it was less evident. 
Horrible smelly job!!

[Al Housman]

It looks like it's leaking on both sides of the tank!!
On the filler side there is seepage around the seal which may also indicate a corroded filler pipe. Given the cost of a new steel filler pipe I might just have a word with a friendly welder to tidy it up. The seal is no problem.
On the left side it's seeping from corrosion on the circular plate which is part of the fuel level sender. A range of options and prices. I have read somewhere that it's worth reusing the seal as the original is of much better quality. 
Does anyone have any thoughts?
Thanks

[Aidan]

Got a used sender here, maybe also a new aftermarket one, not sure
if the seal is okay then reuse; I have new one (BW) here if needed

[Al Housman]

Just to draw this to a close....
A visit to Aidan (he's only 10 miles away) and we(he) sorted  an original,well-used sender which only had surface corrosion. He took it to bits, cleaned it up and tested it and all looked good.
Removing the damaged one from the Doka involved putting it on ramps under the let side to give better access and to aid syphoning the 9 gallons of diesel I had in there. It took a while to clean all the crud from round the wiring loom there .
Actually unbolting the sender (8 off 10mm nuts ) was a pain because the bolts had clearly not been touched since 1990 and no socket would grip until I realised that the sockets all have some kind of an internal bevel and don't grip properly. 30 seconds with an angle grinder flattening the face of a socket magically transformed everything and all nuts came away easily.
Fitting the replacement was a doddle. The seals were fine and new nuts did the trick.
For about a week the fuel gauge refused to budge but I was happy as I could at least get back to work delivering logs but then a few days ago (probably because I've been up an down some pretty steep hills!) something must have moved as the gauge showed what I thought it should.
So all good. And big thanks to Aidan for not only sorting the sender but also for giving me the confidence (not something I generally lack!) to sort it.
And the original diesel leak was as a result of a pin hole of corrosion in the round plate.