I own a property which consists of a ground floor lock up shop and a self contained first floor flat, housing one person.
My Environmental Health office of my local authority has just told me that I must provide fireproofing between the shop and the flat ie. the floor of the flat and the side wall of the stair well.
However they seem unable to tell me what legislation demands this and how I should actually go about doing it. I get the impression that they are unsure themselves of the regulations. Can you advise me please ?

You are subjected to the Regulatory Reform (Fire Safety) Order 2005, enforced by the Fire Service, Building regulations, enforced by the Building Control department, Housing Act, enforced by the Housing department and the Health and Safety at Work Act, enforced by the Environmental department, the last three are the local authority.

When you converted the upper floor to a flat did you get building control approval because under Approved Document B (Fire Safety) volume 2 you would have needed a minimum a minimum of 30 minutes fire separation between the shop and the flat. Your completion certificate will be proof of that.

When you completed your fire risk assessment you would have needed to provide a minimum of 30 minutes fire separation between the shop and the flat. The reasons you do not have it I do not fully understand unless you have not abided by the regulations.

The way to achieve the fire separation is by methods of construction, for instance a stud wall with 12mm plasterboard on each side can achieve 30 minutes fire separation and floors can be upgraded by similar methods.

I would suggest you ask for their comments in writing so you know exactly what they are requiring.

Check out, http://www.planningportal.gov.uk/buildingregulations/approveddocuments/partb/bcapproveddocumentsb/

https://www.gov.uk/government/publications/fire-safety-risk-assessment-offices-and-shops for more information.