Aiden should register a Caution against the land in question. By submitting a plan of the land to HM Land Registry (HMLR), Aiden will be notified if anyone makes an application for first registration in respect of it. This ensures Aiden is aware of any attempts to register the unregistered land adjacent to his garden. A Unilateral Notice or Restriction cannot be registered against unregistered land as these entries are only applicable to registered titles. 

Key Point: A Caution against first registration is a mechanism used to alert individuals of any attempts to register unregistered land, ensuring that their interests are protected.