Ok , bought ute dedicated gas, has had a light hit in front, it would fire on first try for about 3 seconds then nothing, just turns over, fuel cut of switch in left hand kick panel is in , fuses all good, pulled air box of and tried, backfired so I'm thinking there's gas there !! Any suggestions

numerous possibilities -scan for codes -
does smartshield light go out when ignition turned on -
check pcv valve not broken or disconnected due to back fire (both ends of hose)-is air box ok for same reason -
check all hoses connected to & under manifold and check ducting for cracks and secure fitting especially one at mixer (cracks underneath where you can't see it) -
check balance hose between air filter and converter is connected properly and has no cracks (can be hard to see with out removing) -
check for spark (remember your playing around gas) possibly use timing light and see if it fires -
has it had plugs changed -.9mm gap -
try lpg gas bottle in throttle body while some one starts (remember your playing around gas)-
iirc. check for fuse on left side under bonnet for gas -
check connectors near air box are not corroded -

Have a look at the idle solenoid, or blocked idle jet, or the secondary valve stuck in the convertor, the reason being when you turn the key to the start position the engine cranks and gets a crank signal, the pcm is putting battery positive on your two tank lock offs and the liquid lock off at the convertor. Also the pcm is supplying a ground for the primer solenoid because it will run for a couple of seconds before the priming shot runs out, so that tells me the convertor is getting liquid and vapour.