Steven Green
A test to see if the line of neutral is an issue would be the first port of call i.e. If the cables are loose or the socket is faulty. If this fails to sort then a end to end test must be carried out by a electrician as this is issue could be extremely dangerous and is seen as a c1 fault - do not use. I advise you get a competent electrician to check especially if the sockets seem to be in succession on the ring I.e. The socket are next to each other and on the Same ring.