Winter Villa sitting

That is a kind offer but the laws in Spain state that if ANY payment is received for use of a property for tourist or leisure purposes, which your visit would come under, then the property must have a Tourist License and the the owner must declare the payment and pay due tax, even if only a nominal rate that only covers utilities is paid.  So unless you can find someone prepared to let you house-sit entirely at the owner's expense, ie entirely free of charge, you will need to rent a legally licensed holiday let property.  Most owners that do winter lets have a special cheap deal for a winter two month let, although the increased cost of heating may mean that next winter it won't be as cheap as in previous years. Whilst there are many websites advertising legal lets you would get a better deal going direct to owners, so try the smaller websites that allow you to message or email owners rather than booking through the best known ones at the going weekly rates.  You can also search the forum rental page or look on social media - facebook has a number of holiday rental ad pages, but ask for the License number if not displayed (under our regions regulations it should be displayed).  If caught renting an unlicensed property it isn't just the owner who can be fined (heavily), the renter will be evicted immediately and possibly even fined too - it does happen.  An unlicensed property is also uninsured.

Thank you for the information. Rather naïve of me I must say. Having owned property in Florida and France I should have known better. 
I’ll look to rent a full compliant villa via various websites on the internet.