Vauxhall astra mk3 wont start?

Hi, my car wont start. Thought it might be plugs, changed plugs yesterday. Still wont turn over, electrics work i.e. lights, radio, wipers, cl. when turn ignition the power seems to cut as if battery is dead, but as i said electics still work, checked battery with meter, reading 12v, any ideas or help more than welcome.