Can anyone recommend a decent affordable septic tank plus soakaway?

[cowboy25]

This is for a shepherd hut I'm building with the aim of renting out to tourists. Sounds like my local planning authority prefer septic tanks to cesspools which surprised me a little. (I was going to go for an underslung tank attached to the shepherd hut chassis) 

 

Also can anyone explain how the system works? Does all the waste water from showers and sinks get combined with sewage waste and go to the tank? And from there a soakaway means treated waste will soak into the soil? 

 

I'm looking at this tank but it seems some what small septic tank soakaway kits - Septic Tank Kits & Septic Tanks UK | SepticTank.co.uk but I guess if its self emptying that doesn't matter.