Many thanks for the tips guys.
I'll try an extended vent line, maybe not as long as Wayne's, to begin with, just a big loop, up to the top of the tank and down again.
On my other jets the vent pipe comes from the top of the tank, up a little and down. No obvious venting. But on this DuBro tank it comes straight from the bung to the fuselage bottom.
I also thought of a small tank, a fuel trap, like a bubble trap in reverse. But I'd need to make sure it didn't fill when refueling.