I live in an old House with old services. I have located the brick-built septic tank which accepts all my bath water and effluence. However I have several cast iron downpipes that disappear into clay drains and when I stick my arm down it feels like round clay pipe. Majority of the downpipes are on the front elevation of the house and the ground slopes away so in my head I imagine a network of linking clay pipes that terminate at a large hole full of rubble. Question is: how can I be sure? Are there any clues in locating it. My deeds don't say anything. I'm asking primarily because one drain is clearly blocked and I would like to trace its route to sort it. Many thanks, in advance.