Air con re-gassed, compressor heard running but still warm, help please

Hi everyone,

Having just picked up my clio 197 i noticed during the test drive that the air con wasn't putting out cold air, so when i collected the keys i went and had the system re-gassed.
Now i made sure that the place i took it to would carry out a pressure test first and apparently it held pressure fine; once gassed the car was run for 20 minutes to ensure the temperature remained constant and it did.

When i went back and collected the keys i noticed the car felt cool inside but as soon as i started the engine the compressor can be heard running but again there's no cool air.
Just wondering if anyone has any ideas, i'm going back to the garage on Monday as they said they'll look at it but don't want to waste my time if they go down the wrong route.

Cheers,
Ry