i have just bought a new door handle for the driver's door on my passatt estate. i would like to know how to change the lock barrel, so my old key will fit the new lock. please help

I beleive there is a phillips head screw in the inside end that holds on a flat piece of metal, pay close attention to how the little spring is mounted under it. After removing that just insert the key and push the cylinder out towards the outside of the handle.

thanks, but i don't have the key for the lock i am fitting, i wonder if i could get the barrel out without the key?

Not easily, it usually will break something to force it out. You can try to break the cylinder apart carefully.