I would appreciate any help anyone is able to give me on this. I live in Spain and I am trying to check the polarity of a wall socket where I plan to plug in a extension block with all the various parts of my hi-fi using an electricians screwdriver (there is an improvement in sound if all the electrical components - amps etc - are the same polarity). I found to my surprise that both sides seem to be live! When I stick the screwdriver into the holes in the IEC plug on the end of the (removable) cable from the extension block I get a light from both holes. Shouldn't I get a light only from the live hole? What could be the reason for this? many thanks in advance.
there is an improvement in sound if all the electrical components - amps etc - are the same polarity
If there was any improvement, the corresponding equipment can be considered severely broken - because it is not properly isolated from mains voltage. Mordern equipment uses switching mode power supplies anyway, and the secondary side should be referenced to PE - protective earth.
electricians screwdriver [...] I get a light from both holes [...]
Those lie to you all the time. The light is really high impedance - ususally 1MOhm - and can be triggered by capacitive and inductive coupling. Do not rely on them.
In Spain, they have a split-phase power system (similar to the US) in many older houses. That means, both jacks at an outlet are 115V live with a phase shift of 180° between the two. This gives you the 230V common for european appliances, and 115V for aged appliances you may still have at your place.
That's why your checker screwdriver lights up on both poles. They are both 115V against earth.