I have some evidence as to the leak source. A friend was following me out of work. Apparently, when I razzed it round a corner a load of fuel was dumped out the back of the car.
That sounds breather pipe / fuel tank related then.
Ahaaaaaaa. It all makes sense now. That's why the smell gets worse when I corner hard or accelerate hard. It's the Petrol sloshing out of the hole.