you probably need to get it re-gassed, with aircon you need to run it for about 10 mins once a week if you dont use it. if it aint cold enough it usually means gas, so you will need to get it serviced, air con is not covered by a standard service, usually costs around the 50 quid mark i think
My mate at work went to Kwik Fit in his Vectra and they Re-gassed for £50 quid. Apart from the usual "your brakes are worn sir..." routine, which he ignored there was no issue. I off to Kwik fit next month too.