No wonder there was a blockage in this wash room with this curious section of waste pipe. It was all revealed when the boxing in was removed during a maintenance call. An electrician once said to me "Plumbers only need to know one thing; s**t runs down hill". If that's true then whoever fitted this wasn't a Plumber! This is a typical 'bread & butter' day to day type maintenance job for us. It clearly wasn't feasable to simply unblock and leave this as it would have soon blocked up again. The second picture shows the inside of this pipe.
We renewed all the pipe back to the soil pipe connection, clipped it securely and fitted a rodding point for future maintenance. A good maintenance engineer should always leave things in better shape than he found them.