The race is on Ockham Common, near Junction 10 on the M25 - the intersection with the A3. From London, take the A3 towards Guildford. Leave at the M25 intersection, taking the second exit (A3 !) from the roundabout. Before this slip road rejoins the A3, turn sharp left (Old Lane, signposted "Effingham") and then left again into the car park. It's shown on the map below as "Currie's Clump".

OS grid reference TQ 082589