Connecting bath waste to soil pipe problem

[KGann]

I have an old fashioned internal cast iron soil stack that is extremely inaccessible without knocking walls down. There is a pipe that comes off the soil pipe and through an internal wall into the bathroom to connect the bath waste to. I have recently replaced the bath and pipework and am having major difficulties in getting a watertight connection, consider:

Pipe is 65mm (external) 50 mm (internal) diameter.
Where it connects to the soil stack, it is at a slight incline so there will always be a bit of water left in the pipe sitting on the connection.

I have tried a few plumbers merchants to see if there is a fitting to connect this pipe to 40 mm waste, as yet I have had no luck. I wondered if anyone had come across something similar and found a solution? There are more plumbers merchants to try but I am doubtful of finding anything to connect these pipes together. I have also thought about using a setting compound i.e. cement or even grout.

I have tried using combinations of hemp, denso tape and plumbers mait. These work for a while but eventually leak. Any suggestions gratefully received.

 

[longdogs]

What about a good mastic?

 

[Hugh Jaleak]

Whats the pipe made from? Cast or lead? (Or something else...)

 

[KGann]

I thought about a mastic. My concern is that if it doesn't work I'll end up with a right mess that will be difficult to undo.

Difficult to be 100% sure what the pipe is made from. It's a hard material, you can shave it with a stanley but would have difficulty cutting into it. I would say its a hard plastic. As mentioned, its pretty inaccessible so difficult to get a good look at it.