Please help, can anyone advise what can be done before calling out a firm to take a look?

Our central heating does not come on unless the thermostat on the water tank is set at 70 degrees or more.
The Hot water works fine - but is very hot because the temperature is set so high, and the valve for the hot water which is a new one works fine as does the Valve for the central heating since the pipes above and below this are getting hot.

When the heating does come on, the radiator thermostats have to be set to the highest setting (each radiator has its own thermostat) but then they sometimes cool down after a few minutes. We then have to increase the temperature on the water tank to get the radiators hot again.

Can anyone recommend how we can resolve this before we call someone in?
Could it be we have to balance the system? Any advise on how to do this?

We were going to get the heating valve replaced since the new hot water valve may have caused the heating valve to not work properly - ie replace both instead of just the hot water valve? - these are 2 way valves - and the system is a Honeywell 8842.

Any advise would be appreciated.
Many thanks