fuel returning when left

[mpk490p]

since having my weber 45 fitted i have had problems with the fuel returning back when the car is left for a week!
it is taking longer to start because of this and i have to some time take the fuel line of and get the fuel started before the pump can start to pull it (nothing wrong with the pump it is a brand new red top)
the work was done by a top builder but i did notice they mounted the pump where the rear seats go up and meet the floor well which means it is in a higher postion than the original, could this be the problem or do i have a slight leak which is causing me to loose presure hence the fuel being able to return?

thanks
Mark