Connecting plastic downpipe to 3 inch cast

[mal165]

Hi, I'm intending removing all the cast gutters and downpipes with plastic. I have a rainwater downpipe at the bottom of which is a branch taking waste bath water.
I originally was looking at just removing the whole unit but would have difficulty accessing the waste pipe from the bath to the branch since it goes under the floor before exiting.
Alternatively I'm wondering if I remove the downpipe from the branch and fit a boss adapter to the top of the branch to accept the pvc downpipe.
Thanks in advance

 

[Hugh Jaleak]

Remove existing downpipe from socket, offer new downpipe into socket, and I suspect you will find it's a fairly snug fit. Seal joint with some putty or silicone.

 

[mal165]

Many thanks for that info