Filled the SS last night (for the first time in 3 months!!!!) and checked the flap. It was hinged about 5 o'clock. I rotated to 12 and started to pump - only for it to vomit the fuel back out again

I rotated to 6 and it was fine for the first 20 litres or so - but then returned its normal gentle pressure fill. Next time I am filling the car I will play with the flap (ow er missus) to see if there is a correlation. I was wondering whether being very low on fuel caused the vomitting back?
The car took 43l of fuel having oscillated between 3 and 11% fuel remaining - in heavy stop start traffic.
Those 3 months returned just under 20mpg - in contrast the B5s in similar diving over the last 6 weeks has returned 18mpg!!