"electronic thermostats are more efficient but the overall
cost (in my case) would probably take have taken considerably more than

2 years to recoup."

A study was carried out using similar houses and families to find out
what the energy saving was for an electronic thermostat. There turned
out to be none. This was because the ordinary thermostat had such a
large hysteresis that it was used purely as an on off switch and was off
for significantly long times, particularly at night.
Obviously the electronic version is superior (I know I recently
upgraded to one) but don't expect any running cost savings necessarily.