on these old boilers, IF there is a combustion problem it will/could involve dangerous POC's being produced but even so they would go up the flue, if there is a prob with the flue these POC's would spill into the room via the draught divertor, so would not put the pilot out, i would plump for a gas supply prob, when the burner ignites and there isnt enough gas it starves the pilot of gas causing the thermocouple to cool down which causes the pilot to go out, a bit of investigation req'd, when the pilot goes out can it be lit easily and is it a good flame, if so the thermocouple and thermo electric valve inside the gas valve would appear to be ok, it is a bit of a pig to diagnose, checks can be made at the gas meter and appliance ut the gas engineer needs to e there when there is a prob, you could report it to the gas supplier and say (oops lie) that your gas guy has told you the pressure is very low, they will come and check (hopefully change the meter regulator) which could be causing fluctuating gas pressure causing the pilot to be starved.