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Hello I am a solicitor with over 15 years exprience. I will try to help you with this.
Does the neighbour have any right of way or easement over your land?
no not at all it is a private road
it is a private drive which I own
Then you can refuse to allow him access or you can name your price for allowing him access to the sewer.
what kind of price is reasonable to ask please?
That is really fro you and the neighbour to work out. May be you could talk to an estate agent or property valuer about that. If it is the only realistic way he can develop the land then the price might be quite high, if it is just a bit more convenient than doing in another way then it will be less.