Vokera Maxin 24e Lockout?

I am having problems with the Lockout Reset Button illuminating intermittently on my Vokera Maxin 24e, the consequences of which is that the heating goes off and the hot water runs cold. The unit re-ignites when I press the lockout button, but it keeps happening every few days or so, but seems to becoming more regular.

The unit is at least 6 years old, which is the length of time that I have lived in the property.

My very capable electrical-engineering friend has been tinkering with the system this afternoon. He has cleaned it and looked at the ignition board, which looks fine. He thinks it may be an overheating problem as this only happens in central heating mode. I only have 4 radiators running off the system - only 2 of which are finned. Could this be a problem?

Any advice would be appreciated as I'm in fear of being duped by disingenuous central heating engineers!