Hi there,
My journey from Bilbao to St Jean on Sept 12th 2017 was as follows :
Bus from Bilbao Central Bus Station (TERMIBUS :
www.termibus.es ) to Bayonne (Baiona)
Departed Bilbao at 7am : arrived in Bayonne (Place des Basques) at 10am (bus company TRANSPORTES PESA)
www.pesa.net
From Place des Basques in Bayonne it was a 10 minute walk to Gare de Bayonne where I caught the noon train (12.01pm to be precise !) to St Jean Pied de Port (arrived at 1 pm) :
www.sncf.com/en/trains/ter
If travelling this way in August be sure to double check the times of buses and trains closer to the time.
Obviously if getting the 7am bus from Bilbao then this really means staying overnight in Bilbao the night before but it's a great City to stop in and I'd heartily recommend a stay over if at all possible.
I hope the above info is useful.
Travel well and BUEN CAMINO !!
P