I was intending to add a veggie filter to the head waters of the stream by digging out section A in the plan view and lining the hole with flagstone walls and a box liner with the following approximate floor plan dimensions.
That was a job I was not looking forward too.
Then I rescued a 3'x2'x2' cistern from a skip, one ready made veggie filter tank
this was the proposed new layout, T3 is where section B of the first diagram is shown
Here is the plumbed filter tank
Here is the hole for the filter dug, the foundations in and the leveling points, arrowed in yellow, set. The channel is for the 4" grey feed pipe, the cutout, partially obscured by an arrow, is because I boobed in allowing space for the white drain pipes.
The pipe work located in/by concrete
Here is the outlet for the drains from the filter tank
BTW the entire area under the tank is level concrete which supports all of the tank's bottom but I dont have photos of that.
I made a mistake and poured concrete around the sides of the tank. Why a mistake? Because if the tank is flexed and any grit etc drops between the tank and the concrete then that grit is trapped between the tank and a hard surface. It would have been better to use soil around the tank.
The walls of the original streams where soil but I had had problems with them settling so this time I used 2" thick concrete flagstones stood on edge and semi buried to form structural walls for the streams, I dont have any photos of that.
I am posting this now but there is more to follow
, but then it all comes together brilliantly 
and the end product is worth it.
: plumbing 
