Dear Sir/Madame,

As you have no written tenancy agreement it can be understood you have a renewable one month's contract. A month's notice from the landlord to vacate the property would be in order in my opinion. Anything less would be unacceptable.

Besides I am confident that your landlord is up-to-date with his fiscal duties reporting to the Spanish Tax Office his rental's income ...

I must add that under Spanish law both verbal and written contracts are just as valid. The problem is trying to prove and uphold before a law court a verbal agreement.