Starting problem

engine hard too start when left overnight .sometimes nearly drains battery sometimes not too bad. ounce running idles fine drives great . its fairly new battery . fitted new pugs ,oil change & filter . new airfilter still the same friend pugged blue tooth diog in , no fault codes shown. doing my nut some suggest coil pack but surely would miss fire or not run prop, but runs fine. some suggest crank sensor???? bought one from road runner £16 but others say don.t bother if not genuine Vauxhall £78. wont work . ps ounce started can turn straight off and will restart after few turns more or less . driving me crackers