Very bad misfire when taking off

Hi. Got a problem with my daughters Astra. Its got a real bad misfire when starting off or slight throttle pressure (like going round a round-a-bout. Had a full service, new throttle box and throttle pedal (was told there a switch on the pedal that talks to the throttle box). The trouble is when pulgged into a diagnostic machine at Vauxhalls there is no fault code or anything to say if anything is wrong. Vauxhalls advice was either live with it or scrap it ! ! ! Surely if it is a coil pack or E.C.U. it would show up ? Thanks. Any help really appreciated.