From M25 (either direction) take M23 northbound. It narrows from two lanes to one, then you'll see a right turn (Dean Lane) next to a Little Chef. Begin to take this turn, and you will see an area big enough to swing an artic. You then have safe refuge to turn and then pull out into the southbound traffic stream (usually a flood). Don't be tempted to do a U-turn across the southbound carriageway like many people do.
Now take the A23 southbound, passing the M25 sliproad (which you will need to take after you leave the site). Follow A23 for about