The inspection can all be legitimate.
Property owners are not permitted to drain water and sewerage across another property. To do so requires an easement.
Many older drainage systems cross properties and they could have been determining if this was the case.
As far as getting an easement on your property. Only in exceptional cases will a court force a land owner to be burdened with an easement. However you will be awarded compensation for this.
Usually an adjoining property owner will request an easement across your property. You can refuse it, or you can allow it again with compensation agreed between the two parties.
The requesting party pays all the legal and registration costs.