2004 Vauxhall Astra intermittant engine starting failure

For a while now I've been having an intermittant starting fault. It'll turn over but won't fire up. On the odd occasion it'll be completey dead so I'll try it again and it'll turn over again but still fail to start. It can be working fine and then some days it won't start although when this happens I can jump/bump start it or more recently on a couple of occasions I've rocked the car from side to side which has done the trick. It was few and far between to start with but now is more regular up to one or twice a week. I've changed the battery, new plugs and the fuel filter is a lifetime one situated in the tank. I've taken it to Vauxhalls who had a diagnosed a code fault P1614 which I believe is an immobiliser fault but for the 3 days they had it they couldn't recreate the fault and it started every time. I've had the car back a couple of weeks and it's played up twice. The rocking it from side to side did the trick again both times. It doesn't matter what the weather's like, parked on a hill or flat or I've even varied the levels of fuel but there's no pattern to it. Any ideas?