potterton suprema thermostat issue?

I have 2 supremas, 1 for HW t'other for CH. I noticed that the heat output on the CH unit, although turned down to lowest point, was running very hot. Engineer changed thermostat, no change, swapped the CH and the HW thermos, both running at lowest temp position but still v hot, no temp control. they are 15 years installed and serviced by agent yearly both appear to have same problem.
Checked froststat set it to +10C, no change.
Is there an answer other than scrap?