buy the 40kg bags of ballast and cement and carry to back yard ,then mix up in garden its alot cleaner that way,if you npay for ready mix and a concrete pump to get it round the back your be paying a fortune mate,a soakaway is a pit filled with rubble that takes the rain water off your roof and lets it perculate into te ground,if you pick up dog sit off slab and hose down id just let te water run off into garden.