First registration and conflicting boundaries

Hi, We engaged a conveyancer to complete a voluntary first registration of my elderly fathers property and when reviewing the plans before submission we have spotted that 2 triangles of land have been registered by neighbours, making the originally rectangular back garden tapered inwards towards the end of the garden. 
The plot has clear and historic hedges in place and my father is in possession of the original deeds and boundaries. How could someone be able to register part of my fathers garden and what do we need to do in order to reclaim and register the full extent of my fathers property?
Thanks Peter

Hi Peter - land can be claimed/registered in a number of ways. If you want to investigate how the neighbours titles were registered then you can apply by post for copies of the paperwork/evidence submitted as appropriate - your conveyancer can also do that. Search online to confirm their registered information and then apply by post for copies of the application form/filed deed(s) as appropriate - Search for land and property information - GOV.UK 
Practice Guides 1, 2, 4 and 5 explain how land can be registered as well as claimed - Land registration: Practice guides - GOV.UK
If you have applied to register the 'full' extent then the issue will be considered when processing starts. The application will be to register land that's already got a different title or two and that will need to be resolved in some way before a title is granted re your Father's property. You may prefer, in the circumstances, to wait to see how that plays out and what communication there is re the 'overlap' as a result