Poitiers to SJPP

Hi Brendan
There are fast trains (TGV) from Poitiers Central - several per day, it is a major town (fewer on Sundays if you're travelling on the 26th itself). You can get a train from there to Bordeaux and change there to a train for Bayonne, where you catch the train/bus to SJPP. The French rail company is SNCF - all timetables are available on their website at sncf.fr (you can book there too, although if you're going to be in Poitiers for a day or two you could just buy at the station there. You need to have a seat reservation for the TGV fast trains).
Buen camino!

....You might consider walking. Poitiers is on Le Voie de Tours, the Camino which starts in Paris....