Adverts are OK but personal recommendation and experience is worth driving several long kilometres for.  I know of only one workshop that offers a courtesy car - Rueda in Aldea on the Ampolla side on the N340.  The Porche courtesy car is in fact an extremely old Seat-type with braking on some wheels and occasionaly 4 cylinders all firing together.  On the other hand it gets you home while they work on your car !  And generally they're OK but alittle off hand.