Vauxhall Insignia won't start

My sister has a 1.8 insignia

She ran it with no water and it came to a stop in the motorway,

Initial diagnosis by the breakdown company was it had cooked the head gasket as it was venting exhaust gases through the water system,

I have since removed the head, checked for obvious signs of damage to the valves etc, all looked fine,

I didn't have a compression tester to hand at time of strip down so when the head was off I poured petrol in each of the cylinders to see if it leaked through a burnt ring but again this never happened

I have since rebuilt the engine, it's turning over fine but won't fire, I took the plugs back out and they were soaked in petrol, tried easy start down it's throat but still nothing

When it's turning over you can feel the gases coming through the exhaust,

Has no error codes stored

Any ideas would be appreciated incase I am missing something