Heating issue but hot water works

I've got a heating boiler which heats up the water in a tank located in a cupboard upstairs and then from there I get hot water & hearing. I get hot water ok but the heating doesn't come on when turning the thermostat to a higher temperature. I've got a 2 way motorized diverter valve which would out the heating on when I push the leaver to manual mode so I've changed the motorized unit on the valve but the heating still doesn't come on. I have also changed the thermostat and that made no difference either. Any ideas of what else could be the problem here?

You need to use a multi meter and work your way through testing all the switches work in the right order. they should all read 230-240v if they don't then it will be that component

Fused spur live to earth
programmer live to earth
Programmer switched live for heating to earth
Room thermostat live to earth
room thermostat switched live to earth
zone valve brown cable to earth
zone valve grey cable to earth
Zone valve orange cable to earth

Make sure your programmer is calling for heat and your room stat is turned up to the max.

If you're not competent to work on electrics then get someone in to do it.

The power to switch the boiler on for heating comes from a microswitch in the diverter valve head you have replaced. Maybe the diverter valve itself is stiff and won't allow the motor to turn it sufficiently. If you take the motorised unit off the valve and call for heat, does the head operate the microswitch and make boiler run?
Otherwise perhaps there is a fault with the thermostat or programmer?

or as above - fowlerboi who beat me to it has has good advice!