Perhaps I should have said environmental agency offices.

The same rules apply to the whole of England and Wales.

There does seem to be variation to how they are applied though. As you
say elsewhere in this thread, mini treatment plants are becoming the
norm and the one case where such a plant was installed that I am
familiar with involved some *very* heavy "recommendations" from the
environment official despite the fact that there was an existing
(damaged)septic tank and (undamaged) soakaway which could have been
reused.

Not quite that simple I suspect. When applying for a new septic tank or
treatment plant you have to submit test results for the permeability of the
ground. The proximity of watercourses and boreholes is another factor that
has to be taken into account. The greater the permeability smaller the
soakaway that is required. It is highly unlikely that an old soakaway would
come anywhere near the required size and in any case they do gradually
become choked with debris over time. Thus a new one is almost invariably
required. The fact that only a ten year consent will be issued for a septic
tank suggests to me that the will no longer be allowed at all before too
long.