We recommend that external boilers have a frost stat fitted.  Grant outdoor modules come with a factory fitted frost stat set at 5ºC plus or minus 1ºC.  A permanent live is required with a frost stat.
For added frost protection, you should fit a pipe thermostat on a return pipe, set at 30ºC and wired to number 5 & 6 on the control panel.  This will prevent the boiler heating the house unnecessarily.  A bypass must also be fitted when wired as above.