Try the fuel pump relay.
Although you say you can hear the fuel pump turning over my 1.8 vectra (ecotec manual) had starting problems which turned out to be the fuel pump relay because when the fuel pipe was disconnected after the pump no fuel came through. On mine it is located in the fuse box behind the battery where there are 3 relays in a compartment, might be number 11 or 12.
To check if it is just give the relays in there a whack with a screwdriver and it should unlatch them if stuck on and the car will start instantly. A new one is £14 from vauxhall.
Note - I also changed the crank and cam sensors but that didn't fix it!!!!
Andy
May 2007