Read the HSE website pages, relevant guidance and become familiar with the relevant standards. BS1127, BS60079, Energy Institute publication IP 15 Depends that you need to understand about ATEX/DSEAR risk assessment as to what specific training you might require. How much money do you have and how vital is it to you to have the so called training provided by one of the big names e.g. SIRA, ABB, I Chem E to give so called credibility to your training. Small consultancies are just as experienced, they just don't have a pretty letter heading etc. What is your situation to require DSEAR risk assessment training?

The garage/service station is self explanatory. What about the aircraft manufacturer? Are hazardous substances used in the manufacturing process? If so how? Quantities? Ventilation. Are we talking aircraft sub assemblies or airworthy aircraft? What are the hazardous substance F-34/Jet A1 / Avtur or general solvents, oil, flammable gases etc? I have direct aircraft engineering experience and engineering design experience for fuel storage sites and oil/gas/petro chemical processing - onshore and offshore. Are you going to be undertaking Hazardous Area Classification? If you want serious help, send me a message.