For a speed limit sign to be legal the speed limit must be within a round RED circle. If this is the case then it is a legal sign. - I know that for a fact
What I am not 100% sure about is ....
1) The size of the sign - I believe that at the start of a zone the speed limit sign must be a big sign rather than the little reminder signs
2) I believe that if the limit on the road is different to the type of road (e.g. a 60 limit on a single carriage way with street lights, street lights tend to denote a 30 zone - or 30 on a dual carriageway) they must have regular reminder speed limit signs posted i.e. the smaller speed limit signs. Otherwise it is argued that the zone reverts to the limit that the type of road suggests
I am not sure if this is any help but its what I have been led to believe is true