Thanks for the suggestion re SCSI ID (& for autofailover config), but no, the Citrix engineer was only concerned with the UUID. I dont fully understand, but believe that is string is made available to an iSCSI initiator, but that most intiators ignore it. The XEN iSCSI initiator on the other hand, appears to have recorded as part of it's Storage Repository definition. DSS records this UUID in both the volume definition & the iSCSI target definition.

Eg. in this case:
UUID of volume: d64WTF-VB6E-yC3o-1x2o-2EM9-uc9b-EwHZOu
SCSI ID of LUN: VB6EyC3o1x2o2EM9