HI,SOUNDS LIKE YOUR SOCKET HAS BECOME FAULTY OR MAYBE A FAULT AT THE BACK WITH THE CONNECTIONS.SWITCH OFF THE SUPPLY AT THE FUSE BOARD, UNSCREW THE SOCKET FRONT,CHECK CONNECTIONS,IF THEY LOOK OK CHANGE THE SOCKET FOR A NEW ONE.
I WOULD SUGGEST CHECKING YOUR CIRCULATION PUMP OR CHECKING THAT YOUR TIMER IS NOT SET FOR HOT WATER ONLY.IT ALSO COULD BE A FAULTY MOTORISED VALVE IF YOU HAVE HOT WATER ONLY
