my husband is a car aircon engineer. He says firstly to make sure you take your vehicle to a reputable person because if the job is not done right it can be dangerous, not just to your car but also you. It will most likely just need a regas, but the system must be evacuated and placed into a vacuum before regassing, for how long depends on type of vehicle, a zafira is for at least a half hour. also gas must be weighed in because a precise amount must be placed back into the system, as too much can blow any weak spots in the system and to little will not allow the high/low pressure switch to operate, a zafira gas weight also depends on if it has 1 or 2 evaporators, so can be 650g or 900g. before regassing make sure they pressure test system with nitrogen not air, making sure you have no leaks. other faults in brief.. expansion valve can stick not allowing gas to flow or too much to flow, receiver dryer or accumalator can be knacked or pressure switch can be down not allowing compressor to kick in. hope this helps, sorry it's so long