Vostro 3750, will not start even with battery removed

Hi all my dell vostro 3750 will start via mains power with or without the battery removed. Any help would be most welcome. When the battery is out and the power removed via holding down the on/off button and then replugging the mains lead back in the laptop will not start. The blue light from the jack is on but it does not show a white light on at the front and it will not start.

When the battery is in I get 4 amber and one white light in a repeating pattern so I presume this is to say battery is dead. However it does not explain why I cannot use the mains supply?