ST1 or adverse possession?

Stuck4name

Hi everyone

We are intending to purchase a property in England where sellers have part of their garden which is not on the title plan. They have sent us a filled ST1. I am now wondering whether they should submit the ST1 to Land registry and/or apply for adverse possession or would it be upto us to apply for adverse possession after purchase of the said property?

If its upto us to apply for adverse possession, do we need to wait for further 12 years before we can apply for adverse possession?

Also, can we actually proceed with purchase with just getting ST1 from buyers?
We are buying with mortgage from Halifax if that makes any difference. Should we or our solicitors inform Halifax?

Thank you in advance for any response or guidance. 

Land_Registry

All depends on how long the ST1 is for - if less than the required years then you can’t register the claim until you add your years to get there
Yes re advising Halifax as they need to decide if ok to secure loan against registered part without including the unregistered bit of the garden 
https://www.gov.uk/government/publications/adverse-possession-of-1-unregistered-land-and-2-registered-land 

Get legal advice as to ensuring the ST1 states what’s going to be needed to support such a claim in law