We have wooden stairs leading to a mezzanine floor which also has a fixed escape ladder in a warehouse, there are metal goods in cardboard boxes on wooden pallets stored under the stairs and mezzanine floor, also loose plastic items, my concern is that flammable goods are stored under the stairs, is this okay and legal in this particular situation?

It is acceptable, dependent on the fire risk assessment and because of the storage below the mezzanine there should be the means to provide early warning (fire alarm) or the means to extinguish any fires, before it spreads to the mezzanine (sprinkler) which will ensure person can escape safely. 

Much appreciated thank you.

No prob.