Plumber Job - Avondale, WC, 7500

My son lives in a granny flat on our property. His toilet, shower and sink is blocked and we cannot get it open. Our insurers have send a plumber to have a look but he could not resolve it. A second plumber visited and used a jet machine to clear. Still no luck. He gave us a quote to lift he paving, search for a manhole to access the drain and clear. Worse case on his quote is to cut out a length of pipe exposed after lifting the pavers, cleaning it out and fitting a new section. He will end off by installing a rodding eye for inspections and to clear any future blockages. I need a second opinion / quote on this job. My son has to exit his flat, walk around the house and come in to use a bathroom or kitchen. It also smells increasingly bad in his flat as we cannot dare to run any water - the toilet simply overflows if we do.