I doubt you will get Environment Agency consent and building regulations
approval to do it. A fully installed subterranean system cost me £7,000
last
years including removal of the old system

This is what I reckon as well. I can see the EA saying no initially but in
theory what I propose is improving what is there and they are already
happy
with. Instead of the Klargester going into land drains its going through a
whole other system which is designed to deal with raw sewerage. So all in
all its going to be better and safer than any single system. Bit
unconventional admittadly but if they are happy with both systems
individually then I shoud be able to convince them (I hope).

£5-£10K is what I expect it will cost all in. I've been quoted £750 inc
VAT
for a 1000 gal. Klargester so with pipework, dig, install etc should be
reasonable.

I should make it clear I was talking about a domestic plant serving a three
bedroon bungalow. You should also remember that The EA will now only issue
septic tank consents valid for ten years from the date of issue. This means
after ten years they could force you to replace it with something less
polluting. For that reason alone doing the job with a proper plant would be
more sense long term. A properly functioning septic tank is recomended to be
emptied every two years.

Peter Crosland