LK 3 way mixers valves mix hot and cold water to give the desired temperature. They let you adjust the temperature supplied to your heating circuit which helps preserve the heat in a thermal store for longer, and used for domestic hot water they avoid the risk of scalding and reduce heat losses in pipework.
These electronic mixer valve controllers constantly monitor the flow temperature and adjust a 3 way mixer valve accordingly to give the set temperature.
Installed on your flue pipe a flue stat is an accurate way of switching your load unit pump or heating pump on when your stove is on, and switching it off when the stove has gone out.
A remote tank thermometer is a neat way of being able to tell how hot your accumulator tank is while you are beside your stove and means that you do not have to keep going out to check how hot the tank is.