I have found conflicting information on this topic, some sources state that all premises including industrial/factory must have an annual gas safety inspection, while other sources state that factories are exempt and covered in the factories act.

In our building we have 2 x gas fired 40kw heating boilers and 1 x gas fired space heater for the factory floor.

Any advice on what our legal obligation is would be gratefully received

I have a feeling the original question stems from an action on a Fire Risk Assessment. 

When undertaking an FRA the assessor will ask for a Gas Safety Certificate for any gas system onsite. For a residential or commercial system, they will only accept a certificate issued in the last 12 months by a GasSafe Registered engineer as being compliant. For an industrial system, they still expect a documented annual safety check but are more flexible about the need for the 'GasSafe Registered engineer' requirement as 'competence requirements' are not prescribed to the same level for industrial gas engineers. However, it is common that the same Certification to be provided for industrial systems when outsourced to a third party. If out-sourcing you would look for an Industrial GasSafety Registered company. 

Legally all systems have to be maintained and periodically checked to ensure they are safe. The issue of required certification is secondary.

Hope it helps,

Tony.