S Hood is spot on regarding RIPA. To sit outside watching would be classed as 'directed surveillance' and it certainly would require permission. However, to turn up at any reasonable time and enter the premises is covered nicely under the RRO. Even if it turns out to be a domestic premises once you have entered, there is nothing in the RRFSO that says you cannot enter to see if the RRFSO applies or is being complied with.
There is nothing stopping you interviewing guests, it is unlikely that they would be required to attend court as a written statement could be taken/requested instead. Even a written record of a conversation between you and a guest, if written down properly in your own notebook, could be acceptable. If the 'guests' won't assist you then you can't do much about it, but if they 'assist' but knowingly lie to you (to any questions pertaining to the fire safety order) then they are committing an offence. (Albeit one that we are unlikely to take any action over)
If it is not being used as a domestic dwelling then I would get a prohibition/restriction notice on the upper floors until the issues are sorted out. (It might be reasonable to allow the ground or even the first floor to remain in use)