you should have a vacuum put on it(not a household one!!!) to test for leaks and then the gas is put in if all is ok, theres also a lubricant which can and in my case did cure small leaks from dry seals. it cost me 40 quid and is working very well.
i also repaired the small holes in the condensor which has stood the test of time so far, so to get my system up and running cost less than 45 quid, the tins of gas at alfrauds are about 39 quid!!
it really aint worth messing around with!!!
i used pug parts in rotherham, its just a local firm, mainly peugot mechanic but aircon is the same for all cars.
most places will also try to find the leak if there is one.
you would be well advised to give the system a quick check over visually, look around the top corners of the condensor usually tiny pinholes. fill them with the type of liquid metal filler/crack repair rated for aircon that you can get from most motorist diy stores. make sure you thoroughly clean the area tho!!
let me know if you need anymore advice on the condensor. although a new one would be the best option, it is certainly possible to sort without the need for replacement!!