Hello, I am a solicitor with over 20 years experience. I will try to help you with this.
I can't see any reason to think that you have lost this land from what you say. Your neighbour will gain no rights to your land from secretly using it as a dumping ground.
They would need to have been using the land for access continuously for 20 years to even think about gaining a right of way through prescription (long use).
Thank you for your advice. I feel relieved that