1/ Is the battery in good condition? Get it load tested, not just checked for volts.
2/ check the leak-off pipes from the injectors, any leak will allow air in to the system. If you fill the fuel tank and park the car facing downhill does the car start better?
3/ Failing starter motor, on a number of Vauxhalls the front bearing goes, it still spins but not quite fast enough and drains the power.
4/ Heater plugs are not used unless the temp is very low, about 4 or 5 deg. C I think, so less likely to be the problem.
fred
November 2009