Hi Chloe - I don't quite follow the post wording but it reads as if there is a small piece of land that's not within the red outline for either you or your neighbour's title plan. If so then it's not included in the registered extent for those two titles.
You can search online (by map) or by post to confirm if the land is registered or not. I would recommend the postal option as we complete the search for you -
Search for land and property information - GOV.UK
If you want to know if the land is yours/or someone else's then apply by post for a search of the index map and provide a plan that is very clear re the land in question. Use a copy of your own title plan to then mark the land on
T marks are not an indicator of land ownership. Such markings are often used to explain who is responsible for maintaining a specific boundary feature