Anyone know how flame detection works in (Potterton) boilers?

I've got a 1998 Potterton Profile boiler. It has a single electrode which provides the sparks and also does the flame detection. I've read somewhere on the Internet that you can do flame detection by measuring the current that passes through a flame between two electrodes. Rectification of an ac current removes the possibility of false detection due to sooting because of some ionisation effect.

Anyone got any detailed explanations or circuits?

Thanks,

Paul