There are a couple of things about your post that raise flags with me.
1. Its not normally the responsibility of the septic tank supplier to have the site correctly assessed. That's the job of the owner.
2. The septic tank installers can only install the pump and pipe to the percolation area that's permitted by the planning permission. Again its the responsibility of the owner to instruct them with regard to where the dispersal pipework is situated. Unless you retained them to obtain the permission how were they to know? In the present case did you not give them a copy of the planning permission drawing and the assessor's report to work from?
3. Communal percolation areas that are kept separate from a combined treatment system are foreign to me. You either have on septic tank and set of one percolation areas per dwelling, keeping the required distances away from houses, streams, ditches, wells and other percolation areas or you have a group scheme with its own attendant percolation areas.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.