You first need to identify why the tap is leaking. If the tap drips when it is turned off, it means the washer attached to the jumper valve needs replacing. If you get water flowing or oozing out between the tap body and spindle or out from under the cover plate when the tap is turned on, it’s most likely the O-ring that needs to be replaced. Either way, the tap will need to be dismantled, so it’s worth replacing both while you’re at it.
Gather your supplies
• Tap washer repair kit or individual parts
Turn off mains water supply, then turn on tap you are working with and let water run out. It helps if you also turn on a tap that is at a lower level, such as a garden tap. Unscrew button or screw that holds on the handle, remove handle, then unscrew or remove cover plate. This exposes the body and spindle. If there is water in top of spindle and outer body is wet, O-ring is leaking.
Using a spanner, loosen body and unwind it. Remove body washer that remains on tap base and pull out jumper valve, which has washer attached to its base.
Wind spindle out of body as if you are turning tap off to expose O-ring, which sits in a groove. Either prise off the O-ring or cut it with a utility knife.
Wipe everything down so it’s clean, then roll on a new O-ring. Wind spindle back into body so it’s completely in (tap would be fully on).
Place jumper valve in body (or in end of spindle) and place new body washer on base. Grab tap body and wind on. Tighten with a spanner.
Reinstall cover, handle and button, then turn off tap. Turn on mains water and gently open tap to let out trapped air. Turn off and test for any leaks.