I used to be a Fire Service Inspecting Officer and in my Brigade, what you describe is nowhere near the threshold for a Prohibition Notice. It would need a risk of fire (poor compartmentation, dodgy and untested electrics) as well as an excessive travel distance.
On its own, an extra 3 metre travel distance on top of the arbitary 18m travel distance in the guides would not be sufficient for an Article 31 prohibition. I would have to justify getting a very senior officer off the golf course or probably out of the 19th hole and an extra 3m TD is not enough
Fire safety is all about time. Detecting the fire as early as possible by fire detection, holding the fire back by compartmentation, providing safe escape routes in which peole can escape quickly by providing emergency lighting and restricting travel distances.
How long would it reasonably take an 'average' person to travel that extra 3m (under 10ft!!)? The provision of a new escape route would be disproportionate
Is your single staircase a 'protected route'?
That is,
1) Is it fitted with fire detection and manual call points? (please say yes if its L2)
2) Are there FD30 fire doors with self closing devices onto the staircase? Are the walls around the staircase 30min fire resisting
3) Are there very few ignition sources? Perhaps limited to socket outlets, lighting and EL only?
4) Are combusitible items limited in the staircase? No sofas, bookcases, or reception areas (with or without a PC)?
If so, you have created a place of limited combustion - a protected route. Travel distances are measured to the protected route and not to the final exit. Is this less than 20m???
More on travel distances here
According to ADB 'travel distance' is:
'The distance that a person would travel from any point within the floor area to the nearest storey exit, determined by the layout of walls, partitions and fittings.
Where a storey exit is a final exit, or a doorway that gives direct access into a protected stairway firefighting lobby or external escape route
More on protected routes here
https://www.designingbuildings.co.uk/wiki/Protected_stairway
Please take into account, the post I have written has been compiled from details in your post and without sight of the premises, plans or any further information. So tread carefully and get your fire risk assessor to justfy his FRA rather than leaving it to you to consult a solicitor. That sounds like a very cowardly action