FSOs roll has not changed since the Fire Safety Order was introduced, their role did change in 1997 when the Workplace Fire Precautions Legislation (WPFPL) was introduced.
Previously, under the Fire Precautions Act, the owner/occupier had to do very little. The FSO would inspect and then tell the owner/occupier exactly what to do to comply withy the law. They would then reinspect periodically to check that all was well.
Since the WPFPL was introduced in 1997 the responsibilty for determining what was required shifted from the FSO to the employer. The employer had to carry out a fire risk assessment to determine the necessary measures. The FSOs role was to police this and if problems were found they could serve an enforcement notice or prosecute.
The Fire Safety Order changed little in terms of enforcement. Now it is the Responsible Person, and not just employers, that have to carry out fire risk assessments to determine what is required. The FSO polices this by auditing premises using a risk based inspection programme. If they identify failings they may serve notices or prosecute. Usually things can be resolved by negotiation without the need for formal enforcement action.
As far as the role of BCOs and FSOs when building work is done nothing much has changed. If you want to build you first go to a BCO or Approved inspector with your proposals. When they are satisfied that the requirements of the Building Regulations are complied with they consult with the FSO to identify anything that may be further required to comply with fire safety law. All the FSO really needs to do is advise you to carry out a suitable & sufficient fire risk assessment.
FSOs have the power to enter any premises to which the order applies at any reasonable time to make sure the law is being complied with.