Solved the problem. Looking at the Trace ETC I noticed that ZOS was issuing the "JES WAIT DETECTED" message before VM was issuing the "HCPVER575I I/O error" therefore it was ZOS crating the problem. To cut a long problem short you have to EXCLUDE the JES checkpoint data sets from GRS as per the Manual :-( All ok now thanks to Rob for his help.
Stuart
