We have a problem with only a trickle of cold water coming from our kitchen mixer tap. Hot water flow is fine, as is hot and cold water flow from downstairs- toilet sink, and upstairs bathrooms. A few months back a plumber spent ten minutes on it without replacing any parts and this improved it for a while but it has got gradually worse again. There is no cold water storage tank as heating and hot water run off a combi boiler. There is also a dish washer plumbed in next to the sink. Any ideas or suggestions welcomed, thanks.