Most unusually the 6337 Mainboard in an operational e-smith 5.1.2 email Server had a fit of the recurring vapours causing it to 'hang'. I replaced the Mainboard and backed up to a W2000 P4 Workstation, then reloaded with 5.6Update2 and restored from Desktop. Back to normal very easily.
Thanks for a job well done, guys. Compared to previous experiences with 'M' and 'N', this was a doddle.