Isolate the water supply hot and cold to the tap and open the tap this will drain any water from the tap. A useful tip is to put the plug in the waste. This will stop any part falling down the drain, which always happens (Murphys law).
Then remove the handle of the tap and you might need a screwdriver to remove the screw.
This is under a small red or blue bit of plastic top cover. Once the handle has been removed you will need some water pump pliers to remove the cover. Use and adjustable spanner to hold so as not let the tap move you may have to hold it in position.
Now you should have a hexagonal nut twist this with a spanner once again being very careful not to move the base of the tap. Pull out the hexagonal nut and at the bottom is your washer. Replace the complete faucet with a new one.
If the problem isn’t resolved by the advice above, or you’d rather not tackle it yourself, just give us a call on 07855 382 189.