My domestic hot water system has stopped working...... help

I have a fairly typical conventional pumped, sealed, pressurised central heating and hot water system. I have only been in the house for 4 months and yesterday there was no hot water....... The house which is 30+ years old has a modern central heating system installed, which is probably about 6-10 years old at most. Prior to this the house used to have hot air heating...

The boiler is fine and the central heating rads come on fine. When I put the hot water automatic control valve on "manual" (sliding the switch up on the valve) the hot water has come on fine after a couple of hours heating up.... We dont have an electric immersion heater fitted so cannot fall back on that.

Before I call out a plumber I want to try and diagnose the problem myself... the wife has now regained her sanity with the restoration of hot water.....

Am I doing any harm leaving the hot water valve on manual all the time? Presumably this just leaves the valve open all the time?

So what might be the problem - I had thought it might be that the thermostat on the side of the hot water tank is faulty and not calling the hot water valve to open....

Any thoughts? Things to check. If it is the thermostat, is it easy to refit another?

Thanks,
Nick.

Thermostat is the least likely problem. You can check it by turning it up and down and seeing if the boiler starts.