I'm afraid you are going to have to take the tap apart and clean and lightly lubricate the internal working parts, then reassemble. Make sure you turn the water supply off first before attempting any repair, but be careful as turning off water supplies can be tricky, especially if they have not been turned off for a long time. Only a small job but it can be aggro. You should not need a new tap. If you get stuck you may be as well to hire a reputable plumber.
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