Hello, I am a solicitor with over 15 years experience and will try to help you with this.
I am afraid this could be rather complicated. Your soak away has been operating for over 100 years you say and for how long have you been there? Do you know if you have an easement allowing this use of the church yard? If not you may need to rely on having acquired a right to use the land through doing so unchallenged over a long period of time. To find out of you have an easement you should either look at the advice you received when purchasing the property or check at the Land Registry by looking at your title register.