Iain,
In the race, when you get to Rhinog Fawr do not be tempted to turn in the direction of Rhinog Fach. Instead steer at 90 degrees to it and follow small rough paths that appear to take you away from your destination. As you get towards the lower slopes of Rhinog Fawr do a clockwise semicircle to get to Bwlch Drws Arduddwy. The direct line between the two Rhinogs is perilous.
Yiannis
PS: There is no other mountain like Rhinog Fach!