As said, I only removed the back plate and carb, not the head.
BUT watching an unrelated dissassembly vid, I note that there is also carb O-ring. Now, I really don't remember if/what I did with it. Hopefully I put it back in place? Dunno. I really have to remove the engine and partially disassemble it again and see what's what. If it is missing, it's probably where my leak is and not the rear gasket.
Thanks for the tips but I have some work to do so until next time ........