Head gasket with new head bolts ,and thermostat, oil filter and fluids , antifreeze should have cost no more than £80 , Labour 3 hrs £90 , head skim £25 . As a guide so hopefully thats about what you paid. Very strange about the gearbox , it appears very suspect. I would look at the gearbox bolts to see if they have tampered with, or recently undone. They might have stolen your gearbox, sold it to another customer who needed one , or a friend,then put their old broken gearbox in your car.
Or they could have mucked about with your gearbox or clutch on purpose to make you pay them to fix it.
Chris Fidler garage services, Newbury , Berkshire., November 2008