Before applying the WD-40 ala
Elvis.. do this: Take a Zip-top storage bag big enough to fit over the handle in question. Using some typical duct type tape secure the bag and then pour common white vinegar into the bag. The vinegar should cover the valve... especially the stem from the handle through the bonnet nut on top of the valve. Leave it in that condition for a couple of days, remove the bag and vinegar,
then apply some WD-40 and a little force with a pair of what we call Water Pump pliers here in the U.S. They look like this:
http://img2.timeinc.net/toh/i/a/tools/pliers-0 2.jpg
Now, very gently begin to move the handle... wiggling it back and forth in order to break loose the scale holding the shaft.
Here in the U.S. most shut=off valves look like the ones in this discussion link:
http://www.askmehelpdesk.com/plumbing/replacin g-shutoff-valve-under-bathroom-sink-200523.htm l
Notice the nut under the handle on the shaft. Here, it's called the bonnet nut. If you absolutely can't loosen the shaft, then try loosening this nut slightly and then turning the handle. In any of these attempts, be sure you know where the main shut=off valve is for the whole house coming from the street and that its moveable... Best of luck!