What eventually worked was to unscrew the socket from the wall and plug the router into the test socket that is inside the box. I haven't had any problems since then.
then the fault is obviously with your wiring.
I couldn't understand it as we have a separate ADSL socket.
Unless you have a dedicated broadband line that makes no difference.
One thing to consider is the extension ringer wire from BT, it you have no extensions or they don't ring you must disconnect that wire in your main BT socket or it acts like an aerial and makes your line unstable.