2000 Vauxhall Astra starting problem - takes 5-10 seconds to fire

Hi, My vauxhall astra estate takes 5-10 seconds to start. I've had two diagnostics tests done which have came back clear. I've put in a new coil pack, starter motor, changed the oil, oil filter and the fuel filter. I've had 5 mechanics having brain storming meetings resulting in only head scratching. The best way to describe the problem is when the astra esate is really low on fuel it will turn normally but takes about 5-10 seconds before it fires. Any ideas would be welcomed...

Thanks...