Two wire roomstats have the disadvantage ( I understand ) of large
over-and undershoots of the temperature, +/- 3 degrees C being
cited.

Use a digital thermostat. These operate on two wires and frequently have
complex microprocessor based control theory implementations that antipate
better than any analogue contraption. If you install a programmable one, you
can vary the temperature at different times of day, and they are often
installed in convenient locations for programming, rather than at the back
of a dark cupboard full of cr*p.

Christian.