Gary, there are a number of ways to get back.
You could go by train if you want to. You'll have to go to Irun on the Feve or the Renfe line. (
http://www.renfe.es)
Departs Santiago 09h25 and arrives in Irun at 20h32 and costs 19.30 euro if booked online (48 euro general price) And then on the local rail or bus to Biarritz.
The Bus is PESA.net and there are two trains per day:
09h15 arrives at 10h15am and a 14h45 that arrives at 15h40
Better option is to take the ALSA coach from Santiago to Irun and then get a bus or train from there to Biarritz. There are two daytime services but one gets you to Irun after 10pm and the other at 3am the next day. (
http://www.alsa.es)
Better to take an overnight bus so that you don't have to sleep over.
SANTIAGO to IRUN 18:00 arr: 07:00 (next day) 58,68
Then the PESA bus at 09h15 to Biarritz
Times might change in August and there might be extra trains and buses so check the websites nearer the time.