The radiator is connected to the primary circuit which feeds the cylinder, that is probably where the problem is.
Check the cylinder stat is working, then the programmer, then check that your not getting a reverse circulation e.g. taking the heat from the cylinder when the boiler is off, via gravity.
Check the cylinder stat is working, then the programmer, then check that your not getting a reverse circulation e.g. taking the heat from the cylinder when the boiler is off, via gravity.