Hi, thanks for your question. My name is Tony, I can answer this for you.
Did you give rights of access to the neighbour in any agreement or deed concerning the land and the right to have the septic tank on your land?
not that i am aware of
Okay, then he cannot come onto your land without your permission. If he does this, it would be trespass and you would be entitled to issue an application in the county court for trespass, seeking an injunction to prevent him doing it again and damages by way of compensation. This would be done in your local county court using an N1 claim form.
He can't obtain access under the Access to Neighbouring Land Act 1992 as this relates only to situations where the neighbour intends to carry out works on their own land, but in this case, he wants access to your land to service something on it. Without your consent, this would be unlawful on the neighbour's part.