This is exactly the problem - having observed Iain's 101 coming down a steep hill and loosing litres of fuel on a steep decent the fuel filler is lower than the highest point of the tank (which was full). The leakage is a combination of the fuel sloshing back and forwards and the filler being a low point on steep descents.
I have the same issue with my standard setup when the tank is completely full but my rubber seal on the filler cap is not the best.
Iain also has a leak at the tank itself where the rubber filler pipe joins the tank - not sure whether the clamp needs tightening or if there is perished section. From a safety aspect this leak is more of an issue due hot exhausts etc.
The easiest fixes are to fix the leak under the vehicle and put a new seal on the filler cap so that fuel cannot run back out - however there will still be a bit of fuel sloshing back and forward.
Failing that some sort of none return valve might work. At the filler on Iain's 101 there is a mod that takes the filler dia from about the standard 3" down to 2" in the filler. I know a Jag E-Type filler will work well as it has a spring loaded flap inside it that allows fuel in but not out - but these are a bit hard to find.
Failing that, move the filler to the rear wheel arch but that will take a bit of work.
Good luck with it.

