So is the notice valid? Or do i have to accept the points?
1. Yes, it's valid. The offence is simply exceeding the speed limit, not exceeding it by x mph.
2. No, all you have to do at present is name the driver. If it was you, you may be offered a course, and/or a fixed penalty (£100 and three points). You do not have to accept either, you can go to court and defend the charge.
The lack of a quoted limit is not a defence. In any event, the NIP clearly states that you exceeded the limit on a restricted road. Such a road has a default limit of 30 mph.
Unless you have a valid defence, you would be well advised not to go to court.