My system comes on during the night or in cold weather when not asked for?

Check the minimum room temperature setting on your Smart Plus controller; this should be a low-temperature setting e.g. 8°C to 12°C. If the minimum setting is too high e.g. 18°C, then when the room temperature falls below 18°C during an off period, then the room stat will bring on the boiler and keep the house up to 18°C when the house should be allowed to fall to a much lower temperature whilst no heat is required.