Form ADV1 or FR1

Hi, there is some land adjoining our house and when we asked for a search using form SIMR, it came back as:
"No registered estate, caution against first registration or application for a first registration is shown on the index map in relation to the Property.  We therefore hold no records in respect of the Property."
We have had sole use of the land for over 12 years, as did the previous owners before we bought the house from them.
We would like to claim and register the land as ours, with it included on the freehold registration of our house.  Should we use form ADV1 for adverse possession of unregistered land or form FR1 for a first registration?
Thank you.

Hi Nick - form FR1 and see our PG 5 for guidance 
Adverse possession of (1) unregistered land and (2) registered land where a right to be registered was acquired before 13 October 2003 (PG5) - GOV.UK 
The law re such claims is a very complex one and you need to satisfy the legal requirements before tackling the registration ones

Land registration is complex, designed to protect legal and financial interests in property. There can be significant consequences for any error. Please consider seeking legal representation. Before making an application without legal representation, it is important to consider the benefits of using a conveyancer.