The Ardèche is beautiful throughout the year, but be prepared for holidaying French families during July and August at the riverside hotspots. Our favourite months are September-October: the air is still warm, you can comfortably swim in the gorges, and explore on foot or by car without the crowds. October sees chestnut festivals, too.
We also love May-early June: perfect for hiking or biking, bird watching, wild flowers and 'frog concerts' (as our reviewer notes), though do be aware that rivers can still be in full-ish spate in early May.
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By air from the UK: carriers to Clermont-Ferrand, Nimes, Montpellier and Marseilles include British Airways, easyJet, Ryanair, Flybe and Jet2. Flight times are about 2 hours from London, 2.5 hours from northern England.
Within Europe: try easyJet, Lufthansa and Air France.
By train from the UK: From Lille Europe (connections with Eurostar) or from Paris Gare de Lyon, the TGV races down to Avignon, Clermont-Ferrand, Nimes, Montpellier and beyond. Journey times from London, changing at Lille Europe, are: Avignon: 6 hours, Clermont-Ferrand: 7 hours, Nimes / Montpellier: 7-7.5 hours