Do you just take the ferry to Bilbao and ride across from there? Doesn't look like it's too far away... Fair play for riding those routes two up - my G/F loves being pillion on the bike, has done some off-roading with me and is really keen to go do some more. That looks like a great spot for it.
I obviously missed this the first time round. Having seen your pictures in the 'prospective front page' I was compelled to read the report... stunning scenery.
I think it's hard enough to know what tracks are legal in the UK, and I can speak the language and read the maps... so hats off to you