I have a Ideal Classic FF 360 and only the Pilot light works the boiler?

I have a Ideal Classic FF 360 Central Heating Boiler, when i put the heating on the Pilot light lights and I can here the fan but the boiler doesnt light.

I drained the system down over the last few weeks, to clear all the sludge out of the radiators and replace the Expansion tank.

I have then refilled the expansion tank and cleared all the air that was in the radiators, this I have done a couple of times after putting the pump on.

But the boiler still doesnt fire up, I have read a few things on here and they point to a lack of pressure in the boiler.

Please asvise as a need to get this sorted as i have no heating or hot water now.

Thanks