When I turn my kitchen mixer tap on it is very stiff. When I want to turn it to run cold it is very difficult to move and water has now started to come out halfway up the tap not the water outlet. This only happens when I turn it towards the cold water. When I run the hot water the water supply seems slow. I have tried WD40 on the tap. Do I need to get a plumber in. Sorry I'm not much good at DIY but will have a go if you think it is easily do-able. Thanks.

It would really help if we had a picture of your mixer or a brand/model of it. Single handle kitchen faucets are most frequently a cartrdige or ball valve. Yours may need replacing of some plumbers grease. Can't be more specific without info on the faucet.