Hello again Emmet,
The reply from Angulero confirms the train from Santander at 4.10 p.m. The FEVE usually leaves on time but may lose time along the way! In going to Pendueles you will need to walk some 13 kilometers to Llanes. The E9 coast path runs along the top of the cliffs, so from the station just head north when you can. If you can ask you need the "Senda de la costa - numero E9 say ' e nueve' (e as in bed). If you cannot find it I think the Camino runs along the old N634.
I think that I would be inclined to stay on the FEVE for one more station to Vidiago, that will cut your distance down to 10k which will probably be far enough at 6.00p.m. Again, the coast path lies to the north of the village. If I remember there is a 'playa' (beach) near there, so that is the place to head for.
Blessings
Tio Tel