Austin Big Seven starting problem

[Thowdfella]

I have mentioned this problem before but never managed to sort out the problem. The carb is a Zenith vm-4 which appears to be in good condition with no worn spindles. It has now got to the state where it will not start at all. I have done a valve grind; there is no sideways movement on the pistons; the compressions are all around 70 when turning the engine over on the starter motor; the timing has been checked again and again and ignition laser indicators on the plugs are all  flashing. When the head was off we checked the camshaft timing which appeared to be correct. but still wont start. Last night when I was doing a last ditch attempt to start it - I forgot to turn the ignition switch on. When I did turn it on and tried it - it eventually kicked into life!
To me this suggests petrol starvation so its back to the carb. on previous occasions when it wouldn't start, I have removed the float chamber - filled it to a higher level than what was in there and then it started. Can anyone help / I am using fresh fuel and priming with the fuel pump lever and its a new pump which gushes fuel out .