I have a Honeywell st699 programmer controlling hot water / heating. It is an older model with the brown backplate. Recently the time on the clock has been randomly changing sometimes twice per day. I will reset it to the correct time and realising the boiler hasn't fired up later I will check the time on the clock and it will be hours different. Once I reset the time the boiler fires up. I know a power cut will reset the time but this is totally random. I assume a new unit is required So my question is can I fit a newer version st699 to the existing backplate. Would changing the unit battery have any bearing. Many thanks for any assistance.