every few minutes the cistern starts filling again for no reason,then ?

The cistern is supposed to fill
It should stop filling before it overflows
A float on an arm is lifted by the incoming water
As the arm moves it bears against the inlet valve and turns it off.
The piece that does the business is a small nylon screw with a lock nut
Sometimes the screw works loose and you get an overflow.
The usual solution is to wind the crew in towards the inlet valve - and do up the locknut.

Flush the cistern and observe the mechanism operating in the area of the inlet valve.

Peccavi
May 2010