You have 2 alternatives.
You could offer them a personal licence to use your cess pit on the basis that they pay for the use and contribute to any maintenance based on the number of occupiers or you will block within , say, 21 days and they can make their own arrangements.
Whilst the first option is the line of least resistance there is the possibility of disputes in future in respect of maintenance bills etc so getting them to make their own arrangements is less hassle in the long run.
I suspect they have been told they have an easement of necessity but they do not have any implied right to use your system as they had a system before and can make their own arrangements. Using your system is not a matter of necessity.
I hope this helps if there are any further points please respond and I will be happy to reply.
I would go for the 2nd option too, it will save trouble down the line.
You have already given them a 6 month period so I would go directly to 21 days or 28 if you feel more is appropriate.