Car: Travelling from the UK it is about 9 hours from the EuroTunnel terminal at Calais. All 'Autoroute' until the last hour and the tolls, as at October 2016, come to approximately €52 each way.
Rail: There is a night train from Paris to Najac with a change at Figeac about an hour before you arrive in Najac.
Plane: Fly to Toulouse-Blagnac International Airport and hire a car. Najac is 75 miles (120km) north east of Toulouse-Blagnac (about 90 minutes in normal traffic) and a very beautiful drive through the Gaillac vineyards.