Most likely the final drive ratio is lower on your new gearbox than it was on the old one. On older Vauxhalls the SRi models often had a lower overall gear ratio for a better performance through the gears. The compromise of course was a higher engine speed for any given vehicle speed. No petrol or diesel engine is capable of producing high torque at low speed. Your car's gearbox is designed to make optimum use of the engine's maximum torque throughout the rev range. I'd speak to your garage again, as the overall gear ratio must be different.....

Thanks for the prompt reply Railroad...will speak with the garage again...regards Berni