From
allow the minimum energy efficiency setting programmed, and in addition the lowest setting is 5 deg C (frost setting).
the temperature drops the valve will reopen to maintain the temperature set, this will cycle continuously within that set period. valve never closes the lowest the valve will be Minimum it will set is to 5c difference, it cycles to maintain that temp
I'm not quite sure about the wording of that last paragraph, but the above is pasted from the FAQ.
Chris