Valiant Turbomax 282 DHW not very hot?

I have had a 282 for 10 years, repaced diverter valve, diaphams, micro switches etc. The DHW supply has never been that hot even when the thermostate is turned up to 90c. The central heating has always been good. The DHW heat exchanger is clear and the water restricter is clear, but the funny thing is, when you turn the DHW on, the exchanger gets very hot, but the heat does not transfer to the DHW supply which at the kitchen sink is only 36 inches away. Any one else had this problem? Know how to fix it? Cheers, Paul.