All To cut a long story short bodge it and scarper have worked or previous owners terrible diy skills . The soil and waste has been leaking for god knows how long and have ripped a internal wall down to find leak. It is where the plastic goes into the cast. Looking for some advice on how to repair
A McAlpine DC1connector will take you from plastic to cast iron, you might need to cut off collar on cast iron. https://www.screwfix.com/p/mcalpine-dc1-bl-drain-connector-black-110mm/4255v
Have a look here at timesaver,you should find the parts you require. https://guttercentre.co.uk/Saint-Gobain-PAM-UK/Above-Ground-Drainage/Timesaver-Soil
Better photo. Darn naff building pipe into wall, but it was done a lot years ago. A DC1 should fit straight into the cast iron pipe where white plastic pipe is,the best plan would be replace the lot with plastic.
If they've tried to connect a BSP adapter into that, my guess is the thread is chewed. When they dip CI, it leaves a residue in the thread. Not a prob metal to metal but it kills plastic. You need to re-tap the thread to clear out the dip before screwing in the plastic.