2004 Vauxhall Astra astra failing to start

I've posted this problem before but I think I'm nearer to tracking it down. For almost 12 months I've had problems starting my Astra. It was few and far between to start with where the engine would turn over but not quite start. After a few tries it'd start. It became more regular (once every 2 weeks on average) so I changed the battery but still does the same but again even more regular. hen this happens now I have to jump start it. It'll taake a few goes but does turn over a but quicker and fires up. I've also changed the plugs and had a code read which showed nothing. There didn't seem to be any pattern to it, all types of weather, different levels on fuel in the tank or could be standing idle for days and start fine or a couple of hours and wouldn't. I've now noticed that if I park nose down on a hill it seems to start ok. On the flat or nose upwards is where the problem is but has started on the flat ok many times. Just stopped for petrol and the forecourt is on a hill so I was pointing nose upwards at the pumps. Was only there for 5 mins max and stuggled to start after filling up but did after a few go's as there was probably still enough petrol in the fuel lines getting to the engine. Would this problem be to do with the fuel lift pump?