When selecting; bear in mind that there are 'thermostats' and 'programmable thermostats'.
Programmable thermostats allow you to select different temperatures at different times of the day. Incorporating time and temperature control in the same unit avoids the need for a separate programmer / timeswitch.
Basic Thermostats provide temperature control only [as per the one have now, and the example posted by denso13] and do require an additional programmer / timeswitch.
You can still choose a programmable thermostat, but if you do, the existing programmer should be set so that the heating is permanently on 24/7 and the programmable thermostat will then take over full control of the heating.