My mother-in-law had a similar problem the other week when we went to
visit her. She said that a friend of hers had taken a look and said
it needed a new washer. So I nip down to the local hardware store and
buy a few washers (different sizes as I didnt know what size hers
were). When I took it appart it was the small retaining clip that had
worked itself off. The clip is located under the shroud that covers
the tap body.

Alan