Or park in the car park at SU159699 and take the track along by the racing gallops and approach Devil's Den from the North.

From that side you can see the southern end of Mothers Jam on Fyfield Downs, and can see the Devil's Den a long time before you actually get there - look to the left and down from the racecourse.

Make sure it's not too wet though - the final approach that way can get a bit muddy!