Here are the updates,
The engineer replaced the PCB but I have noted that it is not eidentical to the original one, but boiler not firing up, then he diagnosed that the spark generator in defective, come back an other day to replace it , we got spark and burner comes onbut we now note that when the hot tap is open , the boiler ignite as it suppose to and works fine , but when the hot tap is closed, we can hear a click inside the boiler assuming this is the water flow ****ch, the red LED on the RHS of the gas valve goes off but the burner stays on for a further 10 to 15 sec, I can see the temp going up and when it reach 70 degre then the burner goes OFF.
He thinks it is the DHW sensor the cause of this, but I think it is something else or the PCB is the wrong one.
So he is cuming back again to replace the sensor !!!!