When I remove the fuel cap there's always a vacuum, but recently it's got alot worse and fuel sprays out as the cap is removed

Today, it spluttered as I was driving as if it was running out of fuel, but there was plenty. I got out and had a look and there was fuel leaking out the back. I removed the cap very carefully and I could hear things relaxing as the pressure was released.....

There must be a breather blocked somewhere, but I don't know where to start....

There's a small nozzle just inside the fuel pipe which can be wiggled??
