hiya.. another update.. contacted Surrey Rolling Road who then directed me to Abbey Motorsport in Surrey.
These guys said theyre 99% sure they can fix the prob.. they fitted loads of kinetix cats to Z's recently and said they never fit perfectly, need to be cut and re fitted as they put too much pressure on the Y pipe flexi parts which then crack with time. At same time theyll test my cat sensors to make sure theyre functioning correctly.
Im gona use your new Y pipe phil and let them cut and re fit my kinetix cats.. (i decided that i will keep them but this is last attempt to fix it otherwise back to stock cats.. fingers crossed this is the solution).