Split System Air Con icing up at night only..?
I am assuming that this unit is a HEAT-PUMP. If it is, check with the manufacturer as to the lowest an ambient temperature this "heat pump" is supposed to run in. If that temperature is say 20 degrees, then the heat pump should shut down at that temperature and heating will continue on the 2nd. stage heating system. If it is "all the time" then okay we need to check for reasons for the icing of the coil. The outdoor coil of a  heat pumps, when running in the heating cycle will ice up, under certain conditions, but should defrost when the ice builds to where it impedes the heating capacity. If your unit does go into the defrost cycle but does not properly defrost the coil, then there is a problem with one of the controls that initiates the defrost, senses the amount of icing, or terminates the defrost cycle. When operating properly the ice will be defrosted off the coil at any outside ambient temperature. Now I have a couple questions: #1- have you ever witnessed the defrost cycle? If so, was all the ice defrosted off the coil when the heat-pump terminated the defrost cycle and the outdoor fan started running again?  #2 Do you live in a snow area and if so is the outdoor unit elevated off the ground as it should be? I used to set my installations at 12" above the ground. If the servicing company can't get this unit to defrost properly then you need to find a new servicing company with a tech that knows Heat pumps. They are a different kind of animal and can stump even knowledgeable service techs and factory reps.