I thought the following cautionary tale might be of interest to others.
Having just completed installing new sockets in our bedroom I checked them with a polarity tester (the orange UK sort with red and green LEDs which checks polarity and earth connections etc.).
It showed that all the sockets had "reversed polarity". I couldn't understand this as they were all wired according to the usual "normes" as shown in "Comme un Pro" with the live on the right (ie clockwise from earth when viewed from the front). I did a bit more checking at the tableau which showed the main supply was the same. Great confusion and not a little concern.
THE ANSWER
I had used the tester in a UK to French adapter instead of plugged into the UK socket wired to a French plug which I had made up for the purpose but was suing elsewhere so it wasn't to hand.
The UK to French adaptor has the earth connection at the bottom so when the tester is plugged in it has to be inserted into the French socket with the UK earth at the bottom thus reversing the live and earth pins. Simples!
Fortunately I hadn't changed everything before I tumbled to the cause of the confusion.
A WARNING The double sockets I bought from Brico Depot in Bordeaux last year (an own brand double socket) are incorrectly marked with the "L" on the left and the "N" on the right when viewed form the front. Follow the convention NOT the marking. Hopefull they have spotted this and have colour coded the newer ones.