Pretty much by definition a dual carriageway has a central reservation. If there's no central reservation, it's not a dual carriageway -- it's a single carriageway road.
Was there a system of street lighting in place? If so (and no signs to the contrary) then it's a 30 mph limit. If there's no system of street lighting (and no signs to the contrary) then it's a 60 mph limit.
You just need to wait 14 days to see if you get anything in the post. No point in worrying now, there's nothing you can change.