[Ghostuser]

Hi Peter
I have a Cossor neon in an HMV radio. It has twin contacts plus a connection to the shell, thus presumably three electrodes. One electrode goes near the top of a potential divider across HT. One goes to a pre-set pot a little further down the potential divider and the third goes into the anode load of a dedicated double diode triode amplifier (strapped diodes) which rectifies the IF signal and amplifies it. The pot is used to set the length of the discharge column on zero signal. I guess everything depends on the balance between these various HT-derived voltages. Since in many sets with directly heated rectifiers HT rises to a high initial value before the other valves warm up, that possibly explains what you are seeing. I would check the emission and bias conditions of the valve driving the indicator tube as well as the HT itself, which may be low if the rectifier has seen better days.
Cheers
Nigel