We have a diffuser tap unit in the Kitchen and the hot water works fine, but only a dribble of cold water comes out of the single pipe when the cold water tap is turned on.

Sometimes the city will work on the main out in the street and get dirt in the main, You can suck up the dirt into your faucets and clog the inlet ports of a faucets cartridge. Shut the water off and open up the faucet. Pull the cartridge and check and clean the cold water inlet ports. Now place a pan over the open valve body and turn the cold water on the flush out the supply and valve body. Reassemble and check. Better now? Tom