There is a thread on this on the Corsa C forum but i thought i would post my story to help others. My daughter decided to sell her Corsa D 1.4 Design and I noticed the Aircon was not working. Just blowing warm air. By depressing the fill schrader valve in engine bay I could establish there was still pressure in the system. I thought a cheap top up from Halfords would solve the problem and went to buy a complete kit, canister and gun for around £80. Upon fitting the gauge to the valve it read full and I was worried i could be doing damage. So I panicked and took the canister back to Halfords for a refund, unopened, which they did. I then took it to my local indie garage who did a £75 service on the system. He withdrew 7g of gas, it passed a vacuum test, and he re-filled with 470g of gas. The system now works perfect, very cold and no more hissing. So the moral of the story was that even though the halfords gauge said it was full it actually wasn't. Get it filled properly and don't waste your money on a DIY option.