This issue has now been resolved. DVC have advised that the failed hard drive has passed all diagnostic tests and has been added back into the RAID set.

DVC suffered a hard drive failure during the power outage. At this stage we don't believe we have lost any data but we are waiting for a call back from the vendor to talk us through how we get it back up and running. Their stated response times are "within 2 days" so DVC is unavailable until further notice.