Anybody know likely cause of this fault please?

Have a 57reg Vauxhall Astra 1.6 with intermittant problem. If the fuel tank is approx quarter full and the weather is cold I have great difficulty getting the car to start in the morning. Took me 15mins to get started yesterday after putting right pedal to the floor and repeatedly turning the ignition. Good job the battery is strong. Mechanic put it on diagnostic but no faulty recorded re electrics etc. Could this be something to do with the pressure regulator in the fuel pump? Has'nt happened with full tank of fuel.