Vauxhall Astra intermittant poor starting lack power

initial problem was neat petrol smell on filling, then overnight sudden flat spot in low gear, then lack of power in low gear particularlly when pulling up hill, but this is all very intermittant. Now added to the mix is poor starting again intermittant hot or cold. Car will turn over will not fire needs three or four times and sometimes will only just catch. Seems to be alot worse when petrol low i.e quater tank,( engine management light comes on then)or just after filling? Using more fuel than usual