The waste pipe that's been concreted into the building wall, and runs out into a cast iron soil stack has a crack in it. It is a part of the soil stack and will have to be cut out/or smashed out. If I was to cut the cast iron below this point, put a pvc branch in with a pvc to cast iron soil pipe adaptor, replace waste pipe from the pan to the new pvc branch and replacing the cast iron pipe, above the cut with pvc soil pipe. Is that acceptable practice? Or should I just replace all the cast iron with new pvc soil pipe? What is best? I found it difficult not ramble as there are lots of little bits that make up my first plan.