crooksey

At the risk of being marked as "contaminated land" in the future. You are best getting it emptied and jetted, once this is done, if there are no connections to it, fill with soil/hardcore whatever.

Get a pumped outlet, have a very shallow drainage field, it will be fine (as long as the water table is not stupidly high). Or if there is a ditch on/bordering the property, discharge to this. Septic tanks are known for not being horrendous in high water table areas and rely on gravity, so the drainage fields are often set far to low.

He is correct.

Your logic is correct, but flawed in execution. A correctly designed drainage field that matches the percolation rate of the ground (during the winter) and sized accordingly to building regs will have no issues dealing with an output from the graf style outlets (twice a day volumes).

Graf one2clean works with the control module controlling the blower and the pump, if an error code comes about it can throw the cycle off and you will have had no aeration. Once its all running again, if it doesn't go away after 5 days, just get it emptied, it won't go away. Always good to keep a spare pump and diaphragm rebuild kit for any sewage plant owners.

nothing in the tunnel, 70mm+ rocks on the bottom if you follow the spec correctly. Its basically just a huge void, water just hits the rocks and drains away, basically a hollow soak-away with an air vent.

You can use infiltration tunnels with BCO and EA approval, its very quick to get it approved. Graf make the infiltration tunnels so BCO usually state they want a Graf plant to go with it.

40m isn't that long. It will be far easier to install a package treatment plant and discharge to the ditch, If it eventually goes into the sewer fair enough. But will save you huge time and headache, and the ditch will also drain naturally for a lot of its run (i imagine).

Personally I would stick with Graf. Nothing in the tank to fail, spares readily available and top class support from Graf.

Thorfun, your cabinet is an additional cost (that most go for). Cabinet placement is something that can make or break an installation. They can be moved (within reason) limited by distance from the tank etc.

No membrane, mix of pea gravel with some coarser stones for bedding/around pipe. Make sure every inspection chamber has a catch pit of at-least 100mm., regularly clean out the silt. It would also be advisable to have the runs jetted every 5 years.

Underground pipes that are not adopted are very rarely queried by building control. You have made provisions for rodding which is good, they will probably just want a pressure test once finished, especially considering the presence of groundwater after such a long dry spell.

I use Ecover everywhere, safe with the kids and no harm to my septic system or drain field. Buy the large 5LT drums online and if I am ever caught short its also in most shops.

Ross, just get the Graf and get the dosing system. You can even fit remote monitors to alert you of any failures. I have the 9PE personally and it is a great system. You can even control the discharge time, to suit. It has loads of capacity in there, for instance, I can empty two full baths, shower, washing machine etc and it leaves me loads of capacity.

Tanks come with a guarantee, if its lost its shape, its on them to prove its through no manufacturing fault. This is why building control exists, they would have had to guarantee and oversee the installation of a new tank is done correctly. I would suggest speaking to the small local builder and his building controller as a first point of call.