The cold water tap on my mixer unit has become very stiff and difficult to turn off -I have replaced the washer this does not seem to have helped -what should I do.

You have overtightened the bonnet packing nutwhen you replaced the washer. Remove the knob and face plate and you will see a brass nut on the stem That's the bonnet packing nut.. Replace the knob temporary and turn the faucet on just until it starts to run. Turn the bonnet packing nut counterclock until you see water seeping out around the stem, STOP! Now SLOWLY and GENTLY start snugging the nut up until the seeping stops, STOP! Open and close the cold side. Work easier now? Replace the plate and knob. Job complete. Tom