hello everyone

my bathroom basin's and bath's waste is going into a 2" CI pipe. a 40mm
plastic pipe was loosely inserted in a metal pipe that was fixed in the
CI socket with lead and some kind of cement. the joint between plastic
and metal was wrapped around with some crap (sorry) that kept leaking. I
decided to do it nicely, removed the smaller metal pipe and cleaned the
CI socket. now it is all nice and clean, but I don't know how to fit a
plastic pipe in there properly. I was initially hoping for push-fit
adapters, similar to toilet pan connectors that go into cast iron pipes,
sealing with fins, but could not find any. one PM suggested putting a
rubber adapter that goes onto the outside of the CI pipe and fixed there
with a clip, but I could not remove the socket, it is too close to the
wall. I cannot replace the whole stack with plastic either.

any suggestions? I read people talking about caulking with some special
cement and rope. although this sounds dodgy, I might have no other
alternative. could not find the actual caulking compound anywhere
though. what's it called properly and can I possibly make it myself?

maybe I could fix the plastic pipe into the cast iron one with silicone
sealant and fill the socket around with expanding foam, then plaster/paint?

the pipe's internal diameter is 50mm. the socket's is ~75mm.

thanks very much in advance.

/max