Is my property licensable?

HMO Licensing

I need some advice on HMO licensing but do not know where to seek it.
I rent out a property to 5 students. The 5 bedrooms are on the ground floor and the first floor only.
On the first floor there is a stairway that leads up to the loft and that room can be used by the tenants as storage only. There are no beds in the loft room or furniture. Does this make my property a 3 storey building, which requires a HMO licence? If it's a 2 storey house with 5 tenants, in this particular part of the city, a HMO licence is not required.
The problem I have is that I've received a letter from The Shared Regulatory Services stating that they have inspected the house and identified that it is licensable under part 2 of the Housing Act 2004. Consequently the Council believes that an offence may have been committed. I may be prosecuted for this offence and I may face an unlimited fine.
I have also been invited to voluntarily attend a formal interview under caution at the City Hall. If I choose to attend, the interview will be tape recorded and conducted in accordance with the Police and Criminal Evidence Act 1984, codes of practice. I can also choose not to attend and to respond in writing to written questions.
Where do I go from here? Do I need legal advice and can anyone suggest a legal firm that specialises in these matters?