RobertTT wrote:

Also, would I be right in thinking that detergents can have a negative
effect on a septic tank?

So, water from gutters, washing machine, bath/shower/sink should *not*
be run into it.

Instead gutters should run into a ditch; the washing machine,
bath/shower/sink should run into a soakpit.

Is this correct?
All effluent runs into the septic tank. The only soakaways are for the
rainwater downpipes. Well not strictly true of course as the septic
tank has its own soakaway. In the yellow pages you should find
companies who will empty it, as said, for about £100, though mine has
not been emptied in the five years it has been installed. Some of my
neighbours use a farmer, who I suspect pumps it onto his fields, they
save themselves about £20, A poor bargain I think, a proper disposer
gives you a disposal certificate. You can get special liquid, quite
expensive, to aid break down of effluent, but the important thing is to
not use bleach or biological washing powders. Also as has been said, if
there is a problem there is a smell. My neighbours has an old concrete
tank, and that is failing, further more the discharge is coming onto my
land. They are not prepared to do anything about it, and no officials
are interested, if I wish to stop it I will have to go through the
courts, not an expense I wish to undertake.