Had a similar fault on my downstairs sockets, I found that a live wire had been 'pinched' between the socket front and pattress box when someone put the socket back on.
We have been in the property for 3 years so it just goes to show how long a fault like this can take to develop.
We have been in the property for 3 years so it just goes to show how long a fault like this can take to develop.