Thanks for the replies. It was only rewired fairly recently so the sockets will prob be protected. However I will add another so if it trips it won't trip all the other plugs too. Cheers!
It doesn't work like that.
Firstly, both devices will almost certainly have the same ratings - 30mA and 30ms. So there is no discrimination between the devices. In practice, what is likely to happen is that the mechanical delay between sensing the fault and activating the trip, and the mechanical contacts actually being open, means that both devices will trip - ie although one may trip faster than the other, it's contacts might not have physically opened before the slower device has operated and started the mechanical process fo opening it's contacts. But as BAS says - nothing is guaranteed when you put devices in series without suitable discrimination.
If if you (for example) found an MCB that tripped on a lower fault current (say you found one with a 10mA/30ms rating) then you still won't have effective discrimination. Unless the fault really were of a very low level, then what actually happens is that you (for example) touch a live part and significantly more than 30mA fault current flows. This is more than enough to trip both devices. So both devices detect the fault, and within 30ms will have tripped and turned off the supply. Again, unless one happens to trip somewhat faster than the other, then it's most likely that both will trip.
Note the bit about the fault current being more than 30mA. An RCD does not restrict the magnitude of the fault current (that is set by the nature of the circuit forming the fault path) - all it does is disconnect the supply within a specified time which has been shown to be "reasonably safe" in a large proportion of situations. There is always a tradeoff - devices acting faster on lower fault currents would probably be safer in more situations, but would be more prone to nuisance tripping which can in itself be a hazard.
Where discrimination is required between RCDs in series, then it's done on a time basis. It's not normally needed in a domestic environment, but if there were a need for two devices in series, then the upstream one would have a longer operating time so that the downstream one could operate and disconnect the final circuit before the upstream device reaches it's time threshold.
So if you already have RCD protection, then adding another one will not (in the general case) add protection. It'll just be something else to trip and be a nuisance.
