I have a brass Barber Wilsons swan neck kitchen mixer tap, which leaks at the base of the swan neck. I have unscrewed the securing nut, to reveal two semi-circles of brass, which fit into a groove around the base of the swan-neck (the part which rotates). The semi-circles are about 170 degrees, rather than 180 degrees, so there is a small gap between their ends on each side. I couldn't persuade the swan-neck to separate from the base of the tap assembly, though I didn't force it, but there did not appear to be anything else resembling a washer.
I cleaned all the parts with some wire wool (there was quite a lot of scale on the semi-circular brass half-rings) and reassembled, but it still leaks when the water flows.
Any suggestions, please?