Martigny is a city in Switzerland.
South towards Aosta
AGIP station at the southern end of the city is a great spot. To get there, simply follow any street to the south, Rue du Levant for instance and follow signs "Italie", "Aoste", "Grand Saint-Bernard". There is a roundabout, so that cars drive slowly; good visibility; lots of space for cars to park. Marion_and_John found a lift in 5' there.
Accommodation and Sleep
Based on local advice, Marion_and_John slept for free inside the Amphitheatre Romain. It is slightly south of the center of the city, accessed from Rue du Levant. Great spot !