Magor to Caldicot Road,
Take the turning for Severn Tunnel Railway Station.
Skirt past the station and follow the road to where it passes over the M4.
Right turn immediately before the road crosses the M4 and there is a small carpark, I'm not sure how busy it is since the toll booths closed!
Alternatively, cross the M4 to the T-junction, to the left it's a dead end at the rifle ranges and usually very quiet unless the shooting club are in action.
To the right I'm not sure.