Septic tanks...

Bloody hell, that's a lot.

How much to dig a pit, concrete the base, bricking the sides with two skins and rendering the inside with a 5:1 sand and cement?

A concrete top, two inspection covers and a load of rubble for a soakaway or let it empty into reeds in a pond and the job's a good'un.

You need to fill the thing with clean water before you put the top on, in case it needs another coat of render. Personally, I'd give it a scratch and two coats. I can't remember how large they need to be but a

That was in the days before concrete rings were in common use.