You're not liable for 2012 anyway assuming you became legal owner after 31st March.
If the liability date for 2013 falls after the property is fully habitable it's definitely liable in 2013, regardless of whether or not it's occupied. If it's not habitable you need to see what actually constitutes a dwelling house for the purposes of the act.
I believe the absence of a kitchen and/or toilet means the property is not considered a dwelling house wrt NPPR, but you need to check, or maybe someone else here knows.