hot water tempreture

Our hot water is not very hot. We have a gravity system which has been in place for a couple of years. There is a programme clock which switches both water and heating. This works. The heating works fine. The water is hand hot, I have two thermostatic valves that switch on the Hot pipe and Central heating. I turned up the thermostat that just sits on the outside of the ho****er tank. Nothing changed. So I replaced this thermostat. Nothing changed. Any ideas welcome....... Thanks