Air con faults

My 06 Octy 2.0 diesel Elegance has been in the garage for warranty work on 4 occasions in the last 5 months re: air con failure.

On each occasion they claim to have either re-gassed/replaced the compressor/traced the leak. 3 weeks after the car is returned to me the air con fails again!

Do you think I'm within my rights to take the car to an independent air con specialist and simply send the dealership the bill?

Any other ideas?

At least it's not been a long hot summer!