I have had to have work done on foul drains more than once - and in one case the drain turned out to be an undocumented council sewer.

If no other properties connect upstream of your property (or on your property) there are few legal problems, and you won't have to have the council install your new inspection chamber (which is very expensive) but it does sound as if the only answer is going to be a big big hole with rodding access at the bottom, and a gas-tight cover on the top.   

In my worst problem the drain turned out to be over 6 feet below the surface so it was a BIG hole!