astra van 2000 not revving up?

I have an astra diesel van, hard to start without easystart, tickover is sweet as a nut, but engine responds very poorly when trying to rev it up, holds back and struggles to pick up, I have replaced crank sensor, air intake clear, turbo not seized. I suspect airmass meter or throttle pot. Any ideas guys as I dont have any test data to check electrics or components. I am also uncertain of engine size, log book says 1995cc but my insurance thinks its a 1700cc, there is a sticker with 1.7 BZ on the slam paneland I cannot see en engine number, any ideas?