Kay Jay. Save your money and forget the gas courses, providers will tell you anything to get you to sign up and hand over thousands of pounds. The ACS lasts for 5 years then you have to sit the exam again. If you want to work on cookers , fires , combis , unvented cylinders ,meters , hand held cpa , lpg. You have to per extra per appliances , which again last 5 years. Your cpa and test device have to be calibrated yearly. To work in the gas industry you have to pay Gas Safe £400 for the first year and £200 thereafter to work legally.
Its a cut throat industry money money money, its a lot to fork out if your not earning.
If I knew what I know now I wouldnt have bothered.
Its the same for Oil firing (OFTEC) course to, only more expensive to go on their register if you want to be a registered installer. However you can service oil boilers without being on the oftec site.