Does anyone know of a Spanish insurance company that offers a multi car policy??

Think Liberty may do it.
Sammy Thomas insurance will tell you.

Direct line

We use Linea Directa

I use Zurich(and it came in cheaper than all the alternatives)through a local agent in Amposta(also a
Driving school,that's how we ended up getting our insurance through her when my son got his licence. She can also sort vehicle transfers and changing of licences. The cars were insured at different times but when we insured the second she informed us next year she'll set up a multicar policy so they are available. Also we put our newly licensed son on a car as a named, additional driver and expected a hefty premium but based on the cc of the car(1.4) it was only €100 extra annually fully comprehensive. Bargain.
Bruixot

Thanks all for the prompt replies, I will start making some calls tomorrow.