If you're out of warranty then you're probably going to have to pay for the repairs.
But I agree �1200 is a ridiculous amount if all they're going to do is change the head gasket.
Main dealers play on your insecurity about local mechanics and charge stupid labour charges.
Ask them exactly what they're quoting you for over and above just taking the head off replacing the head gaskett.
They'll probably tell you that there may have been serious overheating possibly warping the head and it may need a thin layer of metal taking off (skimming) and pressure testing - They won't tell you that this actually costs about �100 at an engineering shop. If you want a real fright ask them what the hourly labour rate is that they're charging you on that quote you'll probably find it's about �80-�90 an hour. - thats about 3 times the hourly rate of a junior barrister
http://money.guardian.co.uk/consumernews/story /0,,1711959,00.html
Ring around a few local independants and ask them roughly what they charge to fix an astra with a blown head gasket. You'll probably start to hear figures closer to those ianess mentions.