Thanks to all; it now appears to be running normally.
I think that the problem may have arisen during the defrost cycle when my
wife found water lying in the collection channel and condensation on the
front of the freezer drawers.
I cleared the drainage tube, pulled the fridge/freezer out and cleaned the
rear coils (they were not dusty), switched it off and left it for a while.
After pushing back and switching on it hiccupped (running noise started and
then stopped) for a while and then settled down.
I know how to change the thermostat and control unit but obviously do not
want to spend money on one if it is the other that is at fault.
When a frost free combi is on does it stop while the defrost cycle is
operating and would that then cause a slight rise in temperature?