No heating but have hot water
I had this recently with a recently installed Tado thermostat and 8-year old Horstmann diverter valve.

The valve was found to be sticking, The thermostat was fine.

Yo can check this by unscrewing and lifting the diverter head off the valve, the turning the valve to the hot water position with pliers.

In my case, the radiators started to warm immediately.  Putting the head back on the valve reinstated the hot water only situation.  Moving the manual override lever on the valve head had no effect.  

A new diverter valve fixed the problem.