Remove the selecting lever and unscrew the mechanism that fixes into the tap body...you'll find that a thin brass peg has broken off so that it can no longer turn the selecting valve...whether you can get a replacement bit however depends on the make. Try a web surf if you can find the makers name - 'Bramham' is a common one.
John, September 2008
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I can answer this question...
My bath mixer tap selector between shower and bath is stuck in the shower m?