When do I start the job as your PA? ;-)
Unfortunately, I don't think I can help on this one. SNCF have their own UK website but it's actually no more than a portal into the Rail Europe site, which shows that the �56 fare only applies to the 0739 departure. (It's �61.50 at 0856 and �69 at 1337). For a journey of 710km (440 miles), that's probably about right. (It's certainly a lot less than the �238 which Air France are quoting for a flight between CDG and Biarritz on that date!).
I've checked SNCF's French site, which refuses to accept that there are any trains from CDG to Biarritz! You can get from Paris Montparnasse to Biarritz for about �35 at 0710 but that's probably much too early for you. The 1610 service, ex Montparnasse, is about �40 but, by the time you'd paid to get from CDG to Montparnasse, you wouldn't save that much.
I've also tried splitting the journey into two sections (i.e. CDG-Bordeaux & Bordeaux-Biarritz) but it doesn't help. The real problem is that the only services on the CDG-Bordeaux section (and most of the services on the Bordeaux-Biaaritz route) are operated by TGV trains. These services are always dearer than the local and regional trains.
Sorry, but I think that you'll have to pay at least �56.
Chris