Glowworm Ultimate 70ff won't cold start properly?

I have replaced the air pressure switch and the pcb and still I have a problem. When the boiler comes on from cold I get a reduced flame height. This is not on the fault finding. It appears that reduced burn "so told by Glow worm" is to do with less than 19mbar at the boiler. Solenoids either work or not and as I've got pilot and full burn (at times) it's not the solenoid. It seems that after a period the boiler heat exchanger reaches a temp and things work till it cools down again.
I was being pointed to the gas regulator at the meter by Glowworm, but if I get a full burn then I am sure that is not the fault. Any ideas???