And can any one advise on the correct instalation of the wiring to the extension sockets.(colours etc)
The colours are not fixed but they do need to match at each end. Open the main box and check the colours aginst the numbers. Replicate this in the secondary boxes. New build is always a problem. I am reliably informed that carpenters can nail a single cable several times per metre.
If all is fine using the socket with the cover off then the next thing is to disconnect the extensions from this and try it with the cover on. If it doesn't work then it is the box. If it does then it is the downstream wiring. In any event it is something for the contractor to deal with.