stevenellis wrote ...

what happens is the rubber inner seal sits on the neck of the tank , as the tank empties it pulls a slight vac on the seal and sucks it into place on the tank neck

Before filling up my C4P4 i push the flap in , therefore distorting the rubber seal allowing air in and enabling the flap to be opened at a single push of the button

My fix long term was to drill a small 5mm hole in the rubber neck seal at the top to always allow air in and to stop a vacum forming in the filler flap rubber seal assembly hope this helps others ..