Without the kit the examiner uses you don't know if the system is gas tight. Last time, four years ago, it took me a number of attempts to get a gas tight seal where the gas bottle hose joins the main copper distribution pipe. I feared it would not be right this time too. If so I have little confidence in fixing it as I'm not good with compression joints, especially when you have to be a contortionist to reach them.
The examiner attaches a manometer to a test point under the sink. The gas bottle is turned on to fill the system with gas at pressure, then off to create a closed system. The pressure is adjusted to the test level by letting off a bit of gas from the cooker. Then we wait for what seems like an hour (actually 5 minutes) to see if the liquid in the manometer tube moves. If it does, we fail. She peers at it incredibly closely. I worry. She tells me not to, she is just short sighted. Suddenly the manometer liquid moves. Aaaagh. No, it's just the rocking of the boat as I move about. Eventually she declares "That's fine then" and I breath a sigh of relief.
A few more checks and a lot of form filling and we've passed. Yippee! Herbie is declared officially safe and I am £188 worse off. A lot of money for less than an hours work and a fancy certificate, but I'm happy to fork out and have peace of mind and no more test for another four years.