I'd formally complain in writing to the car dealership first (CCed to other managers, and give them a deadline of ~ 7 days to respond or you will take it to Consumer Affairs). If the car was not sold with A/C as stated, not fit for its intended purpose, and missing parts (get an independant report from the mechanic), surely you'd have it in the bag. I wouldn't deal with "Dave" over the phone any more.
Unfortunately - Astras are expensive to repair too, and a bit naff in general
Good luck
JB