Hi, Jules,
I have always found that the official German train company, DB Bahn, has the best website for checking train schedules anywhere in Europe. Not sure why that is.
Their site show that for the 5th of May, there are trains from Biarritz at the following times:
7:14
12:51
13:14
20:07
All of these trains will have one change.
Now, I have no idea whether that's correct, because it doesn't show the 14.55 train you found, nor does it show falcons 21 10 autocar.