UPDATE
intranetnsops.dbo.rb_booking
SET
owner = IsNull((SELECT dbTemp.newlogin
FROM intranetnsops.dbo.tempmembersonemail AS dbTemp
WHERE dbTemp.employeenumber =
(SELECT dbMembers.employeenumber FROM intranet.dbo.members AS dbMembers WHERE dbMembers.employeenumber = intranetnsops.dbo.rb_booking.owner)), 0)
Or
UPDATE
intranetnsops.dbo.rb_booking
SET
owner = IsNull((SELECT dbTemp.newlogin
FROM intranetnsops.dbo.tempmembersonemail AS dbTemp
WHERE dbTemp.employeenumber =
(SELECT dbMembers.employeenumber FROM intranet.dbo.members AS dbMembers WHERE dbMembers.employeenumber = intranetnsops.dbo.rb_booking.owner)), '')
Depends if Owner data type is numeric or not.