The law on 'adverse possession' is complicated but it basically requires someone wanting to 'grab' your land to have used it exclusively as their own for 10 years and then to claim it under the adverse possession rules. However if such a claim was made you would be notified and have 65 business days to object.
Simply ticking the box to say that you objected would then normally be enough to block the application.
However that automatic rejection of the application for adverse possession doesn't apply when "the squatter has been in adverse possession of land adjacent to their own under the mistaken but reasonable belief that they are the owner of it, the exact line of the boundary with this adjacent land has not been determined and the estate to which the application relates was registered more than a year prior to the date of the application".
So, in order to ensure that your neighbour can't say that he had a "mistaken but reasonable belief" that he was the owner of the land (and that the boundary has never been properly determined) you need to make sure that you've informed him (preferably in writing, using a 'signed for' service or, better still, through a solicitor) that you own the land and that you want it back.
(See here for more details of the legal position:
https://www.gov.uk/government/publications/adverse-possession-of-registered-land/practice-guide-4-adverse-possession-of-registered-land )
How you go on from there depends upon his response. If he simply agrees, you can work together to find the best solution. If he doesn't you'd be entitled to remove his wall (but I'd still advise speaking to a solicitor first). If you did so you'd need to ensure that you returned all the bricks to him in order to avoid possible prosecution for theft.