Peeing Fuel

Mine does that too. However, only when the tank is all the way full. Gasoline (petrol) expands quite a bit when it gets hot. It is just leaking out through your breather / overflow tube.
That's my theory, and I'm sticking to it!

That must be it, but it's never done it this much before or with so little fuel (although the tank is mostly full I've got about 30 miles in on this tank). It's making my OCD mind wonder.

i think - i might be wrong - the breather hose allows air to enter the tank as fuel level drops, also allows fuel fumes to escape and not buildup pressure in the tank when temperature raises. it originaly goes to the charcoal can (in N America). As the fuel does not expend that much with temp, but it produces a lot of fumes, (it is volatile), it supposed to only be for air/fumes - not supposed to have liquid gasoline in the charcoal can...

now, as Joe said, i think there is a 2nd vent/overflow hose on the tank, but i'm not sure exactly what for....

I noticed mine did it too the other day. The temp was about 94 degrees, tank was pretty full, maybe expansion of fuel to the overflow?