I have Never had to remove a steering wheel on an astra to allow removal of the immobiliser module, some have metal shields around them that prise off easily with a screwdriver,

As david has said, there are 2 x versions, one pulls straight off, but the other type needs the barrel removing which usually means pressing the pin in which you can normally get to by removing the wiper switch.

"OR" you can use a stanley blade or sharp screwdriver around the edge of the barrel and cut off the locking tabs on the module that are stopping the module from pulling off.

H.T.H. Tony.