2007 Vauxhall Vectra

Hi there please can you help me i have a 57 plate vauxhall vectra v6 cdti its only coverd 12500 miles last week i was driving around and i stoped the car the engine was still ticking over i was stood still for about 10 mins when the engine cut out i tried to restart it it started but cut out again then after leaving the car off for a while it starts fine there is no blue smoke or service lights on ect but i have noticed i seem to have lost power and i have noticed when i am in 1st gear and 2nd gear there is a slight knocking sound whilst im driving in these gears but when i put my foot down the increase the power the sound goes just my over all power throught my gears seems to take longer

please help

Regards Daniel