i had mine changed on my astra 1.7 dti Ls last year. It was the Isuzu engine and it cost £180 (fitted) It had a vaccuum pump on it and powered the steering too. From what the mechanic told me you have to jack the bugger up, and remove the plastic guard from the underside of the engine bay to get access. You may have to release the tensioner, then disconnect it from the battery and unbolt. it looks pretty straightforward, as long as you can get to it. best bet is definatly a haynes book though.
andy, December 2009