2004 Skoda Octavia Pressure in the fuel tank.

Noises problem
2004 Skoda Octavia 4 cyl Front Wheel Drive Manual

I have a 2004 petrol Skoda Octavia VRS. Two months ago I removed the fuel filler cap, there would have been about a gallon and a half in the tank, so much pressure had built up in the tank that fuel was forced back up though the filler pipe. Last week, when the tank was three quarters full, I did a journey of 20 miles. The pressure in the tank was so high that fuel was being forced out of the tank and out of the filler cap. After every trip I have to release the cap to let air out. As the air is released I can hear a knocking noise until the pressure has equalised. My local garage can't locate the problem, I have been in touch with the Skoda garage and they have never heard of a similar problem.