Sorry I was a little vague in my earlier reply. As above really, the Astra, Vectra, Zafira and vx220 all have the same z22se engine. The Vectra does have a different inlet cam to drive the power steering pump. It has a hexagon formed into the end of the cam to drive the pump. The vx220 is the only vehicle that has the oil cooler. The cooler is tapped off of the oil filter housing and runs external to the engine.
There is a strong possibility that it could be a cracked thread in the head where the plugs sit. The threads for the plugs run extremely close to the water jacket.
Are you suffering from white smoke from the exhaust?