Hi, kieran!
You have two options (all hours are for today and I think that goes the same for all workdays):
1.) via Santander:
- 4 trains from Fromista (12:03, 17:37, 18:20, 19:37 -
http://www.fromista.com/?v=turismo&c=transporte) to Santander, ride takes 2,5-3hrs,
- but on
www.renfe.com could've found just 2 (12:03, 18:10), price 16,40€,
- from Santander to Bilbao there are many buses throughout the day (from 03:45 to 22:05), ride takes 1:15-1:45h, costs 7-15€.
2.) via Burgos:
- only 1 bus operated by Amaya leaves at 07:30, 2hrs later arrives in Burgos,
- 3 trains from Burgos to Bilbao (10:23, 17:18, 18:35), takes 2:25-2:45hrs, costs 9,15-22,90€.
Hope that helps!
Ultreia!