well If I was you, I let them do the check seeing as its free, and see what they come back with.. but I would tell them not to do any paid work on it yet, as it maybe simple to fix yourself ..
I guess you've checked the gas in the air con system ?
I know that the gas in the air con system usually needs to be re-gassed every three years or so. otherwise the system does not work..
Hope that is of some help to ya
Let us know please
sorry just read your post again, and you say the gas is fine.. Just off to find yet another

could there be a leak in the condenser (radiator).??