It could be faster for you to fly from Orly to Biarritz and get a train to St Jean from there.
Dep: 14:55 Paris, Orly (ORY) Arr: 16:10 Biarritz (BIQ) - France 01h15mn £ 43
If you choose to go by train, the TGV leaves CDG at 13h37 and arrives in Bayonne at 19h48 Costs From £53.50
PARIS CDG AIRPORT(AEROPORT CDG 2TGV) TO BAYONNE -
LEAVING ON Monday 18/08/08 - JOURNEY TIME 6h,11m
Departure AEROPORT CDG 2TGV - 13:37
Arrival BAYONNE - 19:48
Train TGV Interconnexion
From Bayonne you can get a bus to St Jean. The last bus to St Jean leaves at 21h06 and gets to St Jean at 22h18
Bus timetable to St Jean here:
http://www2.ter-sncf.com/Images/Bayonne ... -21351.pdf
Hope this helps.