pretty much repeat of what has been said above but....
just did it on my (WOW.... has it really been a month since i came home?! )
Camino Frances:
booked online March 9th for May 20th departure using raileurope.com
TGV INOUI 8537 - 2. CLASS from Paris Gare Montparnasse dep 10:11 AM arrive Bayonnne 14:05
TER 867309 dep Bayonne @1435 to Cambo Les Bains (arr 14:59) and technically
CAR 405489 from there to SJPdP (dep 15:09 arr 15:52) but they intercepted us in Bayonne proper and put us straight onto the bus and took us to SJPdP so some headaches were saved (although I dont think it would've been too bad... lots of pilgrims in Bayonne so one just has to follow the backpacks, which BTW extends to pretty much the whole Camino "when in doubt"....
)
IIRC I was on that bus at 14:15 and we arrived SJ at 15:30
at Gare Montparnasse - the large blue tableau on 1st floor will display all the info and your train should come up roughly 30 mins before actual departure so thats plenty of time to get to proper platform.
Hope it gives you some more info and eases your mind
Buen Camino