www.gov.uk/permits-you-need-for-septic-tanks/contact
Honestly your system is not working properly,
A septic system is the same as a human digestive system, the bacteria has to be right for it to work, that means unless your system could digest something neither can the tank,
So unless all occupiers of the different homes are being very careful not to use any of the wrong products you are fighting a losing battle.
Does anyone in any of the houses use anything else other than eco products?
Anything like bleach,disinfectants, washing powders?
Because if you have anything going into the system that kills bacteria then the system will never work properly.
The environment agency expect everyone to replace these old systems.
either get everyone to stop using wrong products and stick a dead lamb down there to get the bacteria back or put in your own klargester and at least you will be able to police your own households use of the wrong products.