Carl, I have a similar problem here ....in the middle of the field, there is an old fashioned septic tank for the cottages. Our house has its own septic tank, so one in the field is not ours. Sometimes it overflows, and there is a right mess in the field. The two occupants are not bothered as its not on their garden, and seem to begrudge having it emptied. When it gets blocked they seem to expect us to deal with it!!
Our septic tank is in the garden, and too close to the house in my opinion. It needs moving to a more accessable location for empting. Thats another thing. My uncle who lived here, never had the tank emptied. To my knowledge its been there 30 years and not been emptied. It can't have been as there is no way to it. Never overfills or anything! He never put detergents down it though, and neither do we if we can avoid it.
The older ones should be emptied every 12 months, although the latest models are meant not to need emptying I believe.