I'm thinkin about buying an antistatic mat, as I'll be working with very sensitive PCBs soon.
I've seen several options.
Some cheaper ones, come with a wrist band that attachs to the mat.
But other more expensive ones, have a cord to ground the mat. Is this very important?? Do you have to ground the mat by plugging it to the mains socket??
As far as I'm aware ANY anti-static mat will come with a grounding lead, and you also need a grounded wrist strap as well. Assuming your sockets are correctly grounded?, you can ground them via a mains socket. However, as you don't have your location filled in we've no idea how good your socket grounds might be? - in the UK they are normally excellent!.
It still needs grounding as well, keeping the mat and yourself at the same potential will help to some extent, but a piece of wire and a high value resistor will ground it as well.
If you enter your location in your profile, then we would know where you are!, from what I've heard about Spanish electrical wiring I would suggest you run a wire to your own earth spike!, rather than trust the house wiring!.
On a related note, check your environment for anything that can generate static electricity. I can generate a healthy spark from my chair by just standing up. Ditto on walking a couple steps to a different desk.
I tend to use mats because they prevent boards from sliding around while you work on them - not so much for the ESD reasons. Then again, when handling boards, I always temporarily ground myself by touching something metal before touching anything sensitive.