Selgas
Check and see if you have a tempering valve ( water blending valve ) fitted to your hot water cylinder, the purpose of which is to prevent scalding hot water going to any or all outlets as it blends cold water to reduce the temperature to a preset temp. It is possible and they do begin to fail over time and particually with hard water – the net result is the hot water will slowly get colder and colder until no hot water flows out of the cylinder to the taps/faucets.
Another thing to check is the temperature thermostat on the cylinder may have had the dial knob knocked so the temperature setting has been turned too low – turn it up and se if that overcomes your problem.
