Hi Jean
On the back of the tap body, just below where the swan neck joins it you should see a small hole. in there will be an allen key type grub screw, usually about a 2 or 3 mm allen key will fit it.
you wont need to turn the water off any where just make sure that the taps are turned off. If you remove this grub screw you should be able to twist and pull the swan neck spout out of the tap body. On the base of the swan neck you will se 2 or 3 'O' rings made out of black rubber. Inspect these for wear (i.e nicks taken out of them or rubber dust evident in tap body, If worn or damaged then replace them). If they seem ok apply some grease to them, slot the neck back in and replace the grub screw, you should find the problem solved. Be prepared to have to go to a plumbers merchant (e.g. not on a Sunday) to buy new 'O' rings as they may have perished if the problem has been there for a while. These should cost a few pennies, worst case no more than a few pounds. Remember to grease these once you have installed them on the spout also. Hope this helps
Very best of luck
Pete the Plumber
P.S That'll be a fiver!! hehe
Pete, April 2007