My System Comes On During The Night Or In Cold Weather When Not Asked For (System Has A 7-Day Digital Wireless Room Stat Or 24-hour Mechanical Wireless Room Stat?)

Check the minimum room temperature setting; 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 wireless 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.