In article ,
max-man wrote:
[snip]
I see. Well, last year I had it drained because although the
'crust' started off pretty low down (it was a year drained before
we moved into our current property which is now two years ago) -
but looking at it in about October of last year the 'crust' was
pretty near the top.

Ah! That's where the crust /should/ run.

Just under the crust is water, and below that is the
solid waste, merrily digesting away. The level in
the first chamber should rise until the water level
comes up to the connecting pipe into the 2nd chamber.
The second chamber should also fill until it's water
(only) level comes up to it's outlet. They should
both then stay at their respective levels for years.

The thing to keep a serious eye on is that there should
be no solid matter floating on the surface of the second
chamber and so going down your outlet pipe. This will
clog your easement, which can be very expensive to renew.

--
Tony Williams.