Hi there, this has been my enemy for the year since I bought my car.
If the A/C works after a recharge it's unlikely to be the compressor, this generally seizes at the bearings so would just have completely packed in. It's more likely that you have a leak in an area where it's not visible.
I've had the let-down valve replaced, high pressure and low pressure hosing and the valve/nipple at the filling point. After more months of checks and re-gassing, last week they finally found a leak on the drier, next to the main radiator and not very visible. There aren't many components that make up the A/C system so it's worth asking exactly where was checked. Fortunately I've had my car in over a dozen times and not paid a penny!
Persevere and you'll find it.....
