Anti-roll bar bushes or the favourite would be the drop links could need replacing. Drop links are a common knocking sound from the front end. 30 minute job to replace, very cheap and easy fix.
Have a look in this thread below, read down through it and you'll see my posts about when I replaced my drop links on my wife's Zafira. They are the same for the Astra too.
viewtopic.php?f=62&t=13391&p=1116449&hilit=Drop+links#p1116449
:thumbup: