What's wrong with my Vauxhall Agila heating system???

Hi folks

Over the last several months my car's heating system is nothing like as good as it was previously... in cold weather it takes about half an hour to get the car warm, as very little seems to come out of the vents, and in hot weather it doesn't blow cool air in through the fan quickly enough so we get really hot in the car!

My garage, who are usually very good, don't seem to think there's much wrong with it, but having had the car for three years I can tell the difference is huge...

If anyone has any ideas about what might be the problem I'd be really glad to hear about them - either for me to fix, or to suggest to the garage as a possible or even well-known problem on the Agila.

Many thanks for your help,