Finally got it working this morning. This was after troubleshooting about every combination of things could think of, including uninstalling all software and plugs, reinstalling and building XML over. B slaved to Y still did not work. Then slaved A to Y and worked. Still don't know why slaving B to Y does not work as I have been using that config. for the past year.
Cant think what could cause that I am afraid.
If you want to attach your xml I can have a look through it to see if I can find a problem.
Hood