Hi there...we are from Stockport and did the Plymouth to Santander overnight ferry...April last year.
It took us with a car full of household goods aprox: four and a half hours to Plymouth boarding the ferry around 3.30pm and sailed at 5pm.
Ferry docked at 12 noon next day..we were on our way around 1pm.
We had a leisurely just over 9 hour drive and arrived at Villamartin around 10ish! pm.
We travelled motorway all the way passing Madrid and think we paid only something like €6... stopped a couple of times for petrol and a quick 2 minute snack break...we wanted to get back asap because of all our possessions in the car as it's not recommended to stay overnight in hotels..you might wake up to an empty car!
Anyway it was a stress free journey..The ferry was lovely and the car journey was beautiful! We hardly saw any traffic the whole journey...except near to Madrid as it was home time for most! We would do this route again!
The ferry and car one way cost us just over £300....
Good luck...Hennie