Google strongly suggests that grey water (e.g. from a wash basin in the workshop/shed) cannot be routed to a soakaway, but must go to main drainage or a septic tank.
Haven't checked the fall to the existing drains yet, but I have the sinking feeling that my only option is likely to be a macerator/pump. And that the route will go smack through the area that BC will specify for the soakaway for the rainwater off the back of the house. Unless of course they agree that we can't get the required 5m from all existing buildings and so can continue to discharge the rainwater into the foul water drains.
My original thoughts were to put in mains drainage to the Mother of All Sheds if the fall was O.K., or just run in mains water for a wash basing and drain that to a soakaway.
Looks like the second option may be a non-starter unless someone knows different.
Cheers
Dave R