I can't answer your question, but you do need an easement to go with the deeds as otherwise any future owner of the field in the next 20 years could refuse to allow you to continue using the pipe & discharging to the ditch. It is best to get a solicitor to draw up an agreement suited to your specific circumstances.
You need to be careful what liability you accept. For example, if something went wrong with your sewage treatment plant and you discharged raw sewage into the ditch would you have to pay for any resulting clean up? This is not necessarily a matter of your relationship with the farmer so much as with the water company. If possible get yourself insured against such liabilities.