on the old SP vokeras the fan is 2 speed, low speed (when no demand) to remove residual heat from the pilot, a common fault on these if unserviced if for the pin from the diaphragm to seize therefore stopping the microswitch returning to low speed position and then the fan stops, on these models the fan is never off, so if it not running i would check the microswitch, if you have MI's it will show position, left hand side from memory, if you carefully pull the lever forward the boiler should light proving the diaphragm is jammed.
