After a recent trackday at Blyton, I got black flagged not long into the morning session and was told I had fuel pouring out of my car while cornering.
They said it was a common issue with 7s and it must be coming from the fuel tank breather hose and it was best for me to drain my tank to prevent it.

My fuel tank breather, at the time, came off the top right of the tank, up so it was level with top of the rear tub and then pointed back down and cable tied to the roll cage leg.
I have now just ran it straight up so it goes slightly higher and does not point back down (though I'm not sure if fuel is still coming out or not)
My breather is just a length of fuel hose with nothing on the end.

Anyone else had this problem? and how do you go about sorting it?

Non return valve cheap 100% cfix, let air in, no fuel out.

Ideally you need a 2-way breather VALVE. That lets air/vapour in and out for expansion/evaporation and as fuel level drops but stops fuel coming out. There are cheap one way valves

The alternative is to run your breather pipe, form a loop, then route from one side, across the top of the tank to the other side (sloping up a bit) and then down below the bottom. You can then tip the car anyway you like and fuel shouldn't get out. You may get a small drop if you go from hard left to right or vise-versa very quickly.

I have researched this problem extensively as I too have been black flagged. The answer is actually quite complex despite many who will tell you otherwise. This problem is particularly bad on injection systems where the fuel is returned to the tank warm, inevitable expansion takes place and the fuel will overflow. It is made worse by the frothing that takes place in the fuel when you are racing. Those that tell you to fit just a NON return valve are dead wrong it is a recipe for pressurising your tank with sometimes drastic consequences. It is particularly bad in 7 style cars as the inlet filler to the tank has no length and cannot act as an expansion chamber. It is obviously worse with a full tank. There are two possible solutions.

1) I racing foam filled tank with a long neck and a overflow from the top of the filler. The neck usually comes out at the top of the rear section of the 7 style boot area.

2) An over flow management system. This can take several forms and is present in some modern cars. My Puma gave me the idea. You fit a Mocal valve, it is a valve that has two directions 0.5 bar inlet and 0.75 bar outlet. It is the key element in a fuel management system. You can either fit a carbon filter that will absorb the excess fuel and vent it via a pipe in the normal way or a fuel recovery vessel.

I used an (cheap) alloy catch tank as a fuel recovery vessel with the pipe going into the bottom and the vent pipe going into the top. When the fuel over flows it fills the tank and when it breaths it sucks the fuel back in. There is NO net loss of fuel over the full/empty cycle of the tank using a 1L catch tank. I have tried this for the last season with no more black flags. I have tried it now on two cars with 100% success.

more info: http://sylvabuild.blogspot.co.uk/2014/10/fuel-overflow-management.html

I see, so you don't use any valve on your set up? just simply a catch tank and run the breather from the top of that?

No I use a Mocal two way valve + catch tank

I just use a hose running from the top of the tank to below the bottom with a very narrow restriction when it exits the tank (0.5 mm I think), simple & cheap and it seems to eliminate all leakage as far as I can tell.