Find out who the tenants pay their rent to, sent that company/person a letter requesting the name/address of the owner, stating your powers under the RRFSO with specific regards to identifying the responsible person 27(1)(b), stating that it is an offence to either obstruct an inspector 32(2)(d) (i.e. If they do not reply) or make a statement they know to be false. 32(2)(c) Remember that even a tenant could be guilty of an offence of obstructing an inspector, it doesn't have to be a responsible person or a person having control.
If they fail to reply after a few attempts, go for a prosecution for obstructing an inspector. I believe that a few FRS's do this on a relatively regular basis and succeed in the prosecutions. (Often getting the same person on a few accounts of obstructing an inspector by virtue of them ignoring multiple letters) Whether they would go for it against a tenant that is a different matter.
In these situations I think it is prudent to always go for the owner with regards any notice in flats. (Regardless of who they contract out the management to and the level of responsibilities under the contract.)