You're welcome mate, hope its not too painful.
I can almost guarantee that you will be needing that torch again.
In the end I used an angle grinder to cut off the nuts from the studs (takes ages with little space under jacks), removed the y-pipe, then added heat to the cat flange + stud to hammer the studs out. Think the cat studs have splines, so no threads, so they are easy to heat and hammer out.