Classic Boiler Overheat?

I have a Classic FF 250 that seems to switch off after 75 mins. The LED on the PCB goes out and nothing works for hours. Then the LED comes back and the boiler will fire - again for about 75 mins.

The feeder tank in the loft to the radiators ran dry which caused the problem (I believe) originally but this is now full again.

Is this still the likely cause of the overheat? Is there something else I need to do i.e. feed water into the system somehow?

Thanks