Hi there, I'm struggling to get hot water in my kitchen tap. It runs hot initially but after a couple of litres begins to cool off. Upstairs seems fine and the boiler seems to fire up OK when running the kitchen tap. The kitchen tap is a mixer tap of awful quality. Could this be the problem (Poor Mix?).

Does the water flow slow down as the temperature gets lower? I've seen a supply shutoff (below the sink) that closed down a bit as it got hotter and cut the flow back. I would bet on the poor quality faucet first though. If you run hot water only, what happens? Run hot water for a bit until things start to cool down and then feel the hot supply (below the sink). If that is hot and the mixed water is cool, it is the faucet. Just some quick ideas.

EP is on the right treack. If you have stops (shut off vamves) below the sink try closing the cold valve completely. When you open you faucet on full hot I am betting it will come out fast, furious and hot and then slow down to eventually nothing. You mention no vibration so I am thinking debris in the hot inlet on the valve or cartridge. If you give us a brand or a picture we may be able to help you disassemble the mixing valve