Interesting point, DD......if there was a mobile speed camera team there and someone was zapped doing more than 30mph on the side that states that it is still a 40mph area, then if given a ticket, you would have a case if it went to appeal or court.
Photographic evidence would have to be taken by the accused and sent in to explain why that speed was being done.
The problem is whether in the meantime the sign is changed to the correct 30mph limit as the other side says, and you have no photographic proof to back you up.
I would mention it to the council/police and get it logged and changed to the correct speed limit for THIS reason......If someone gets really hurt, especially a school child crossing the road and the driver is doing 40mph instead of 30mph, which could mean the difference between them being slightly injured or seriously injured, possibly killed... which would be tragic.
I would report it. :0)