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Hi, not sure if I need the cap to be totally airtight as I think it should be able to breathe a little, I guess it's a difficult balance between sealing and breathing,
I don't think I can adjust the pressure applied to the seal, maybe try and find a thicker replace??,,

Maybe. If the main gasket was thicker it might help. There's a few different ways to approach it. First thing that is needed is to make sure all the orings and gaskets are in good shape. There's one on each cap mounting screw and a rubber thing in the middle for venting. It should still vent regardless of the cap pressure. Also make sure that the metal lip your main cap gasket is touching is free of debris and flush. If it's dirty, chipped paint etc you won't get a good seal.

Try pushing down on the cap with your hand. Do you get alot of movement? I was thinking you could adding material to the latching part of the tank. So it fully latches a little further down. It would apply more pressure to the spring and make a better overall seal.

Might be a crazy idea though.

Take a close look at the raised sealing ridge on the tank. This is what the rubber gasket on the cap makes contact with. It may conceivably have nicks and dings on it from being repetitively banged with the gas nozzle when filling. Any tiny deformities/depressions from this sort of metal to metal contact will create tiny non-contact areas between the rubber gasket and that surface of the tank. This will allow fuel to leak from the cap proportional to the damage. In this case (and depending on the severity) you may be able to carefully remove these little "dings" with some fine emery-cloth. Of course the rubber gasket must be in good condition. The vent can be problematic for two reasons. If the nipple on the underside of the cap does not perfectly align itself into the center of the vent-grommet that rubber grommet will block the small opening in this nipple. A new grommet/seal will cure this assuming the nipple is: 1) still there, and 2) not bent/misaligned. The vent pipe itself can become corroded and plugged with rust. My experience with this specific issue: If I filled the tank too high and fuel expanded it had no where to go other then out the key-slot. Hope these comments help point you in the right direction.