Should the mechanic repair my car for free if the job was not done properly?

I recently took my Astra to have to head Gasket done. It was at the garage for 2 days. when i got it back it was still losing lots of oil (burning it..not leaking) I took it back and the mechanic said it was something that happens after the head gasket has been repaired and it was nothing to worry about. so i drove it and kept topping it up for a couple of days.. then a couple of days ago when i went to start it, it wouldn't turn over at all. I looked under the bonnet and the creamy stuff had returned to the oil. I have asked a few friends and they have all said that the head gasket has gone again. Can i ask the mechanic to repair it for free?. Do they have any guarantees on work carried out?