Not totally sure Jenny (but may in near future - but I will be hoping to own a property here)....
I know you can write a will here, and have it notarised and it is fully legal (and registered in Madrid), but it can be relating to your own countries laws even though you live here, and will be executed accordingly. (I think I am right that it is for non property owning ex pats)... There are several firms out there who offer this service (look in freebies, do internet search etc)
Good luck.
PS - Age Concern may be able to help, in fact I think I may have seen something on their Spanish website! 😉