crooksey

Quite common for IC and pipes to fill with crud during the build phase, avoidable but common. Usually always cleaned out, they have used concrete risers and caps, so imagine pipes been installed with a laser level. Jetting and vac out of drains isn't as expensive as you would think.

You only need to replace a septic tank if you discharge to a watercourse (or anything other than a drainage filed). Trouble is many solicitors scare buyers as they (and the buyers) aren't really too clued up on non-mains drainage. You can still install new septic tanks today. If you are selling, I would recommend saying to your solicitor you will get the tank emptied and jetted before the sale completes and provide evidence. If its been in and working since 1983, you have no issues (unless you discharge to a watercourse etc). Also I would state how often tank is emptied and provide evidence of it, compared to waste water costs these days, its actually cheaper to have a tank/treatment plant on a monthly basis than it is to have a mains connection.

Isn't this just a normally/open closed solenoid coil?

because if you add the NRV in that manhole in the house, it would only work up to the "back" edge of the inspection chamber, water that entered the mahole next to the road would still back fill into the IC in your kitchen. If you fitted the NRV i posted into the IC in the kitchen it would be the wrong way round, and effectively block that IC from passing effluent into the manhole in the road. If you are not confident doing it, I am sure a local drainage company would be able to assist. It cold also be that the manhole in the driveway needs replacing with an "air tight" one, this will ensure no flood water enters it. It may be worth getting a CCTV survey just to check there is nothing un-toward. I see you are in Surrey, I am not affiliated with them, but I use "Totally Blocked" for all my CCTV surveys and any patch repairs that need doing. https://www.totallyblockeddrains.co.uk/.

It won't work on the IC in the kitchen, probably flows from there to the one on the drive (easy to test). It would need to be on the last IC before the property, so it does't allow backflow.

In theory, there should be no reason why effluent or water comes back through a manhole cover, unless the downstream runs are blocked. To answer your question, manhole grease should help yes. But I can assure you, if water is getting up through a manhole cover, then more foul air than you realise is as well. Something like this on your last manhole before the road could help: https://pipetek.co.uk/products/retrofit-non-return-valve?variant=31966322917494 This below video shows how NRV's can protect in storm surges:

Adding a non return valve on your drainage run before it joins the surface water (if combined sewer) will stop storm surges causing a problem on your foul runs. You need to clean and maintain them regularly.

Just so you know (in England) this is still technically legal. As long as its not discharging to a watercourse, do not let people in this industry scare you. I have seen tanks from 100 years ago still in use, someone cleaned it out, re-pointed it and added an air blower on a timer. If it was me, I would negotiate the price to reflect this. But in reality, you could move in, have the pipe from the house jetted and re-lined, jet the tank and probably be fine until you get round to installing the new system. With a healthy dose of muck munchers once a month.

Does it need to be pumped, if you are replacing a septic tank, you may (hopefully) get away with a gravity outlet? GrafOne2Clean, Klargester Bio Air. Both good plants, have no media and easy to cleanout etc. I prefer the Graf plants as they have more options for control panels, and secondary treatment etc. But ideally, you want a tank with no media or moving parts (less to maintain).

No one likes fixing other peoples problems, especially when its dealing with other peoples s***, so sadly it won't be cheap. 2 options, pay the money to a reputable firm and resolve it quickly and easily (but expensive, or option 2, do it yourself). If summer rain caused you a problem, got knows what the winter will bring. Get tank fixed, get drainage field sorted, or pump to ditch etc. There is no silver bullet, as you have witnessed installing these plants is no easy task, there are lots of variables. Unless you are very competent and have had drainage field tested and sized correctly, they will cause you problems. My advice would be to sort before November when the rain and winter water table starts making it troublesome to do anything until the spring. Best advice I would give, would be to call the company that empty your plant and ask for contractor recommendations (they may even offer services themselves)

If your building controller is happy with that, then happy days. But if not, move the rainwater soakaway to under the front drive, leave a drainage field in the back for the plant. Or leave as is, and also have a soakaway under the drive for the garage roof and the downpipes on the front of the house.

I disagree with a lot of the comments here, what we don't know are a lot of the conditions on site, percolation test value and another of other things. 4.10x4.50 is a massive soakaway, like huge (13m3) and we don't know the roof size. I agree a klargester is a dated design, but in reality, lots of architects use them as a reference point as its in all the BIM files and drawings etc, something like a marsh or a graf plant would be better. NRV on the outflow is a great idea for all PTP as it stops any back-flow flooding the plant. If you have a low percolation test value AND you don't have a high water table, there is no reason in theory that this will not work. For a soakway of that size, I would personally use attenuation tunnels, as opposed to crates, as these are EA/building regs approved with prior approval. They leave less internal structure for media to cling to, and all water goes direct to soil.

The only time planning is needed is when you are building a new property, or knocking down an existing and replacing it. Under the new dwelling plans a new wastewater plant *may* be conditioned as a pre-start condition, under this you will need to submit percolation tests and drainage design to be approved by the planners. Some times simply stating "new package treatment plant and drainage filed to be installed" and this is sufficient. If you are just refurbishing an existing property, you meet the general binding rules and the plant will only be serving one property, then planning is not needed (but you will need building control). Depending on the design of the discharge of the new plant, you may need Environment Agency approval.

Loads of options these days. Drainage mound, boreholes, larger attenuation tunnels, having a huge drainage field. All are better options coupled with a package treatment plant that using a cesspool. I would recommend approaching a decent drainage firm for some help on site. What is the geological footprint where you are (clay/chalk/sand etc and how far down), and how far down is the winter water table? might be worth getting a 10t digger and digging a deep hole, seeing if you get to chalk/sand etc, or hire a borehole survey to determine if a borehole would be suitable. It is worth noting that many non standard methods will need the approval of the Environment Agency, but a decent installer/surveyor can usually guide you in what would be most suitable/acceptable.

Dig out, mix 6:1 dry mix about 1ft deep, compact and sit chamber on this, backfill around sides and pipes with pea gravel. Rent a tench whacker and whack all out pipe trenches before laying pipes, as a first timer its easy to dig the trench "too much" and then lay new pipes on loose ground, whacking the trenches before laying shingle and pipe is paramount to stop future failure. Fill a good amount above the pipe with gravel, if the ground does/drop settle this gravel will drop down and fill the voids, same principle for below the pipe in the event of clay heave etc.