We have an old septic tank, ie. big concrete box in the ground, half
settlement tank and half clinker.

The tank is well past is useby date and we've just discovered the original
drains are failing etc. I'm looking at a £3K bill just to get the damn thing
emptied due to its location.

Heres the plan... re-divert existing main drain 30m to a 4500l Klargester
style onion from which the grey water runs through another drain to the
original tank (once emptied).

The theory is I'll have a a proper Klargester doing the seperation and the
original tank then "filters" the grey water even further and in theory never
needs emptying again (hope/pray).

Sort of a double tank system with the Klargester emptied quarterly.

BTW , based on the FAQs 180L per day I need (180x70x300)+(180x8x300) = 3.9m
L... since our water is metered I know we only used 1.5m L last year so I
gues this is more accurate? Rain water runs off through land drains.

A BioDisc is out of the question. Have been quoted £40-50K. I can do this
system for under £10K including the emptying!

Any thoughts, observations, obvious flaws?

Ta