Best way to do it John. Break off the pipe collar carefully and use the pan connector as you suggest. You may find that the internal diameter of the clay pipe is slightly too large to give a good seal with the pan connector in which case a short length of 4 inch plastic pipe can be pushed into the clay, then the pan connector. If you use this method you will need to clean the inside of the clay well, and seal the "gap" between clay pipe and plastic pipe with sealant to stop gas escaping.
Hi, I'm attempting a very similar task to this, and wondered if you had any recommendation on how to remove the ceramic downpipe of the WC from the clay pipe. They are cemented together, and I'm worried about cracking the clay pipe if I'm too heavy handed.