How a septic tank actually works
It is a settlement and filtration system rather than a treatment plant. Solids settle and remain in the tank until it is emptied; the liquid discharges through a drainage field into the ground, where the soil does the rest of the work.
Two consequences follow. The tank has to be emptied periodically, which is a running cost. And the drainage field matters as much as the tank — a failed or undersized field is the commonest reason a system stops being compliant, and it is invisible from the surface.
The six items to put in the instruction
That it is in working order. That it has no cracked pipes or walls, failed mortar joints or leaks. That its capacity is sufficient for the number of people who will occupy the property — a tank sized for a couple is not sized for a family of five.
That it is not causing pollution. That its location is known, along with the drainage field. And whether the binding rules are met, including whether a permit was or is required, and whether building regulations approval was needed for any part of it.
Tell your surveyor the property is on a septic tank before they attend. It changes what they look at and what equipment they bring.
Why the cost lands on you
Because compliance obligations attach to the operator of the system, which from completion is you. If the system is found not to meet the rules, bringing it up to standard is your cost, and replacing a failed system is a five-figure job rather than a repair.
That is the whole reason to spend money on a survey before exchange: it converts an unknown liability into a number you can negotiate about.
The rules in this area have changed before and the requirements continue to develop, which is why the answer has to come from somebody who works with them rather than from a document written at some point in the past — including this one.