OK. Progress made. Next job is to see if I can get the pipe run low enough to fit under the new vanity. Otherwise, I'll to try and take of the reducer to go back to the waste pipe.
Technique I used was a combination of your advice, so thanks very much. See pictures attached,
0) Cut across pipe near bottom to bring it back to the bottom coupler\reducer
1) Check if waste goes into sewer pipe. It didn't look like it as pipe seemed to continue around bend underfloor back towards soil pipe on other side of the room.
2) Use a hack saw blade to carefully cut into the existing internal fitting that is to be removed at 4 points around the fitting. Be careful not to cut right through to avoid damage to socket that will remain.
3) Use a hole drill smaller than the outside diameter of internal fitting to remove. Drill out as much as possible. This made a big mess of swarf down the pipe so I would recommend have some kind of rag down the pipe that you can easily remove all the swarf with. Don't try and vacuum it out like I did unless you like fountains
4) Heat up the hole drill bit while still attached to drill with a heat gun to about 150C. Insert into pipe to heat up the remaining fitting. Use pliers, screwdriver to carefully pull\prise away the fitting from the socket. It should be softer and solvent will separate more easily. Repeat for each quarter section, repeatedly heating the fitting as appropriate to avoid too much pressure on remaining pipework.
5) Done