Further thoughts and memories! Air bottles just supplied compressed air to the starter turbine. No combustion along with that. Another option Zantop used at one of their outstations was a large (15 ft long 6 ft cylinder) which was filled with compressed air over several hours. Supply was from a compressor attached to a car engine. This was a one shot wonder. Blew for 20 seconds or so; if you screwed up the start there was no way back! In view of this an Air Start Unit was the standard option at route fields. One foggy morning in Finland I could not get into Helsinki, so went to our company alternate Turku. Before shutting the last engine down I got ALM to check they had an ASU. Thumbs up. As No 4 wound down they asked how many volts we needed from this ASU! So one had to be trucked in from Helsinki over the day. Cost £800 from memory. Happy days