Hi jitterjames,
you come to my rescue once again - thank you.
Indeed it was the issue of too many large xml payloads in the same group.
I simply backed up my files, deleted each group until I found the offending group that caused the issues, restored the backup files, then did some long overdue administration in terms of tidying things up into appropriate groups etc. taking into account the one that had been identified as the issue and hey presto all is now fine again.
Thank you.