2004 Vauxhall Astra starting problems

I'm been having starting problems at random intervals with my Astra for a few months now. It'll run fine for a couple of weeks then every so oftem (becoming more regular now)I'll try to start it and it'll turn over but won't fire up. There's no pattern to it. It can be standing for days and start without a problem but on othere occasions it be be left for a couple of hours and then doen't want to know. The battery and plugs are new and it's been in 3 times for a code read but showing nothing more than a lean fuel mixture. I have noticed though if I leave it parked nose down on even the slightest of hills it'll start fine when I come back to it. On the flat or nose up on a hill is when the problems start although as I say randomly, not all the time. I have noticed that if I rock the car from side to side when it refuses to fire up to get the fuel moving around it seems to eventually do the trick. Any suggestions would be appreciated.