I'm not sure about the Astra, but the fuel filter on the Vectra is located just in front of the rear passenger side wheel about a third of the way under the car, next to the fuel tank. And it was a bugger to get off!!
You have to press two plastic clips down, one on either side of the fuel pipe connecter and then pull the fuel pipe off the end. The one closest to me was easy but, as i only had the car up on an axle stand, the futher away one was a tricky manouvre as it was hard to reach. Once the fuel pipes are off you have to transfer the plastic retaining clips on to the new filter. Then it's just a matter of pushing the pipes back on the new filter again, making sure you've got it the right way round, and making sure the pipes "click" lock back in place properly. It took me about half an hour in total.
Also, i'd advise you to depressureise the fuel line before taking it off. You do this by locating the fuel pipe the runs round in front of the engine on the left hand side. It should have a bend in it with a raised in piece with a black cap on it. Un screw the black cap and wrap a rag around the valve, and then using a screwdriver (or something similar) press the centre of the valve down. Petrol will shoot out, but stops straight away. Replace the back cap and go ahead with your filter replacement.
Hope this helps 🙂
