astra mk4 starter bolts are in a pretty awkward place! Get the car high off the ground with a jack / ramps/ axle stands to give plenty room to lie under car. You dont need to remove anything from the engine, just a bit of patience and blind fishing with the socket ( use an extension bar, makes it much easier) its a fiddle but bear with it, good luck! ps-getting it off is the easy part, much more fiddly getting the nuts back on. Ive done starter changes on my astra g in 15 mins.
andy
July 2011