Express 100 combi boiler does not fire up?

My Glow worm Express 100 combi boiler is nearly 20 years old. I have a problem where the main burner does not fire up when I request heating. The green running light comes on as I had set the thermostat at the required room temperature, and set the timer to ON. I cannot hear the pump turn of the fan start and I checked that it may be a flow switch. I just want to confirm that this is the case.