Hi, we have a fire exit that opens from a workshop into a car park using a push bar. Are we allowed to have a drop of less than 6" on the exit before the car park floor? I am looking through the regulations but I can't seem to find anything that mentions a change in surface height for fire exits.

With outward opening doors, we used to require a meter of level going before you had to step down, I believe it came from the building regulations. 

If should study approved document M but not being totally au fait with most of the approved documents I cannot give a location.

Check out  

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/441786/BR_PDF_AD_M2_2015.pdf and https://www.gov.uk/government/publications/protection-from-falling-collision-and-impact-approved-document-k.