Dont buy kits, take it to a proper air con bloke... because there is a usually a reason you have lost your gas, so if you fill it up again then your just going to loose it again, waste of £40. When a decent air con bloke re-charges your system, he puts oil in the system to which is needed for the keep the compressor running properly and stop dry seals (which cause leaks). Also if you have lost your gas, you probably have normal air inside the system, so you need to suck that out using a vac pump, which you cannot do using a kit. He will also pressure test the system for leaks, and when he charges the system, he will add a yellow die to the gas which will show up very bright under a UV light which makes finding leaks, if there are any, very very easily.
You should always run your air con for AT LEAST 10 mins a week, to stop your seals from going dry and leaking, and keeping the compressor in good condition, most problems with air con are from when people do not use it for a long period of time in the winter months.
Hopefully in the next few months i will have all the kit to do re-gassing myself
my mates dad does it, but no longer has time because he has a new job, and he's training me up, and soon i am going to buy all his kit off him.